IT COMES as a surprise to many that, although Rome was chosen as the capital of the newly united Italy in 1861, it could take up that authority only in 1871, when power shifted away from the papacy. As a result, many institutions and departments of the government of President Napoletano are housed in former papal palaces.

The National Gallery of Art was first created in 1895, incorporating two princely collections separated by the Tiber: the paintings are to be found in the Palazzo Corsini in Trastevere, and in the Barberini Palace on the slope of the Quirinal hill, where the European Convention on Human Rights was signed in November 1950.

The latter grandiose structure had been planned for the Tuscan aristocrat Maffeo Barberini when he became Pope Urban VIII in 1623. It was designed around an earlier building by Carlo Maderno and his nephew Francisco Borromini. At Maderno's death in 1629, Gian Lorenzo Bernini took over. Of the 1549 Strozzi building, only the great garden room survives.

Until recently, highlights of the collection (predominantly from the 16th and 17th centuries) had been displayed in the vast central salone under the deservedly famed ceiling painted by Pietro da Cortona. An ambitious programme of renovation has now been completed. Only the final room, with its paintings by Giovanni Lanfranco and the more recently acquired marble bust of Pope Gregory XV by Bernini, awaits proper labelling. Room after room speaks of the opulence that was Rome's in the first half of the 17th century, while individual paintings vie for our attention.

Most famously, the late Raphael Baker's Daughter (1520), the so-called "Fornarina", who may have been his mistress, holds court in one of the first rooms, while the small head of a young lad, attributed to Giovanni Bellini, offers the very essence of what it means to be a man. His brown eyes seem to search through the light of the late- afternoon sun that irradiates his face, lost in wonder, prayer, and praise.

The portrait of Erasmus of Rotterdam, dated 9 September 1517, was part of a pair with a friend painted for Sir Thomas More by Quentin Metsys. It is hung next to a version of Holbein's 1537 portrait of Henry VIII. Whereas the scholar is rapt in attentiveness as he pauses in his translation of the Epistle to the Romans, the 49-year-old monarch looks out with a steely gaze: More had been put to death two years earlier.

Most visitors will recognise at once the Caravaggio Narcissus, even though some scholars dispute the attribution, as it does not appear in any literature; and his gory Judith and Holofernes. Both are early works, from between 1597 and 1600, and are contemporary with the two oil sketches by El Greco for the Baptism of Christ and an Adoration of the Shepherds, two of three commissioned for a college in Madrid. Here, too, is the Sacred and Profane Love painted by Giovanni Baglioni in 1602 in direct competition with Caravaggio, whose own submission is to be found in Berlin.

The current exhibition in the gallery introduces a remarkable artist whom I had quite overlooked in all the years I have visited Rome. Although Antonio Aquila, known as Antoniazzo Romano, who was born around 1435 or 1440, was rapidly overshadowed by the younger Raphael, who arrived in Rome in the year in which he died (1508), his paintings show a development from what we might regard as late Gothic to the first blush of the Renaissance.

In his early thirties, he painted the remarkable fresco cycle of the history of St Michael the Archangel for Cardinal Bessarion in the basilica of the Holy Apostles (1464-67) in the Roman district of Colonna, where he had grown up. In time, he became the leading reformer of the artistic Guild of St Luke.

His father was a painter and this exhibition, the first monographic show to be dedicated to the artist and surely not the last, explores his working relationship with other Quattrocento artists such as Benozzo Gozzoli, Melozzo, and Perugino.

There are works by him across Rome: he painted frescoes for the Mother Church of San Giovanni in Laterano (one of the few churches that has a double dedication to the Baptist and the Evangelist); and, in the neighbouring pilgrim Church of Santa Croce in Gerusalemme, his fresco cycle of the finding of the True Cross fills the vast apse of an otherwise incongruously flat and lifeless church.

For the Pantheon (second altar on the right), he painted an Annunciation in which God looks down as over a perimeter fence on the scene in the Virgin's chamber; and in the next-door Church of Santa Maria sopra Minerva, another for the mortuary chapel of Cardinal Juan de Torquemada (d. 1468), who chose to be commemorated for the charity he had established for giving dowries to young girls. It is his nephew who reminds us not to talk of the Spanish Inquisition.

The exhibition, staged in the ground-floor rooms of the palace, brings together two dozen of Antoniazzo's works, and also includes later pieces by his sons and pupils. While the quality varies, usually as works from churcheshave never been re-touched or restored, the dignity and poise of his compositions is immediately engaging, and the scenes themselves begin to break the liturgical stranglehold of late-medieval convention.

His models are lively, real people, as witness the sad, longing gaze in St Vincent's eyes in an altarpiece from Montefalco, or the desiccated face of St Vincent Ferrer OP in the votive panel from the Roman HQ of the Dominicans at Santa Sabina (1488-89). In an altarpiece from Milan, the compassion in St Anne's face is almost beguiling as she holds the young Virgin on her lap.

In 1482, he painted a Madonna and Child for the chapel of Sts Pontianus and Eleutherius, the patrons of Velletri, in the cathedral there. Standing behind a low parapet, wearing a midnight-blue mantle embroidered with stars, the Virgin holds the standing Christ Child, who offers his blessing.

Her downward gaze teaches us that we should be demure and thoughtful in the presence of the Christ Child, and respectful in front of all children, who are made in the image of God.

Murals salvaged as early as 1637 (by another Barberini, who was the cardinal patron of the Third Order of Dominicans) from the Dominican convent where St Catherine of Siena once lived show that Antoniazzo was as accomplished in that medium as he was in that of the more refined panel painting.

If an excuse were needed to visit the re-hang in the Barberini of this fine national collection, Antoniazzo Romano more than provides it.

"Antoniazzo Romano: Pictor Urbis" and the newly refurbished galleries are at Palazzo Barberini, Via delle Quattro Fontane 13, Roma, Italy, until 2 March. Open daily (except Mondays) 10 a.m. to 7 p.m. Phone 00 39 06 32810.

www.galleriabarberini.beniculturali.it.