In any discussion of who was the greatest artist ever, it is a fair bet that the putative winner would fall somewhere in those two centuries or so between the birth of Leonardo around 1452 and the death of Rembrandt in 1669. And it is safe to say that Sir Peter Paul Rubens, diplomat and courtier as well as painter, is a contender.

Yet his standing has long been equivocal, even today when he has been enjoying a critical revival. So do we need another Rubens show, with the full retrospective at Lille only last year still fresh in the memory? The enthralling exhibition that opens today at the National Gallery, closely focused on the work of Rubens's formative years, echoes a resounding "Yes".

Rubens was born in Antwerp in 1577. His early childhood was spent in Germany, but on his father's death in 1587 his mother brought the family back to Antwerp, where, in the early 1590s, he began his studies as a painter, first under Tobias Veerhaecht, a landscape painter, then with the figure painters Adam van Noort and, later, Otto van Veen.

So far, so comparatively conventional and obscure, with admission to the Antwerp Painters' Guild following in 1598, and a career as a painter of classical and battle scenes before him. But van Veen had studied in Italy, and in 1600 Rubens followed his example. He was to remain abroad for eight years - variously in Venice, Genoa and Rome but with a base at the court of the Gonzaga Duke of Mantua, and with nearly a year at the Spanish court at Vallodolid and Madrid, sent there by the duke in 1603. Such privileged exposure to some of the greatest princely collections of the day, set inparticular around the work of the great masters of the previous century - Michel-angelo, El Greco, Titian and Veronese - had a profound and immediate effect.

It is this early, essential transition that this exhibition illuminates. It opens with the talented and ambitious but as yet unformed artist in his early 20s, his work still instinct with a northern sensibility and a feel of the 16th century, the figures stiff and with a hard finish: it ends with a master fully fledged at barely 30, working on a grand scale and with the utmost freedom, as much in the scope of his composition and imaginative invention as in his technical handling.

Even in the first two rooms of the exhibition, the speed at which Rubens is travelling is evident. Those early infelicities are clear enough, perhaps, in such works as the early "Judgment of Paris" (c.1599) or the "Martyrdom of St Ursula" (c.1602), or "Aeneas and the Trojans" (after 1602). And while Rubens's drawing is for the most part incomparable in its mastery, here a sepia study of an old man in distress (1602) is oddly unconvincing. Yet already a tiny study for the "Judgment of Paris" (c.1601) is in a different spirit in its freshness, delicacy and freedom, while the huge "Council of the Gods" (c.1602) that dominates the first room, with Juno and a deliciously brazen Venus pleading before Jupiter over the fate of Aeneas, though open in its debt to Veronese, already fully prefigures the great Rubens to come. With the "St George slaying the Dragon" in the next room (c.1606), with its swirling, dizzying movement and its open celebration of soft flesh and firm muscle, he has already arrived.

Rubens returned to Antwerp late in 1608, by which time, at 31, he was among the most sought-after painters in Europe. It is to the Rubens of the next few years, the lessons of Italy and Spain absorbed, that the second half of the exhibition, and pre-eminently the large central hall, is given. It is dominated by two great works of that period, hung in opposition, the National Gallery's own "Samson and Delilah" (c.1610) and the "Massacre of the Innocents" (c.1611). Will the old controversy of the "Samson"'s authenticity revive? The direct comparison with the "Massacre", with which it shares so much not just in the handling but also in the individual character of the models, should have secured it by now. Yet the more tender human sympathy shown in the only slightly later "Roman Charity" (c.1612), of the beautiful Pero giving her breast to her starving imprisoned father, Cimon, matched as it is by a richer, more ample spirit in the handling, suggests a further transition and maturity.

Each of the six rooms has its glories: the magnificently stately Marchesa Doria (c.1605) and a seated "Susanna" (1606), as angrily surprised as ever by the Elders, to keep St George company in room two; room three full of drawings and related sculpture; in room four, the smaller study in oil for the "Descent from the Cross" (1611) and the much later revised version of the "Raising of the Cross" that together are the great glories of Antwerp Cathedral and, to many eyes, Rubens at his greatest. His drawings throughout are beautifully integrated with the argument, hung closely where relevant to the related paintings. For drawing is fundamental to Rubens's mastery, the function not just of a prodigious natural facility but also of long study and observation. He famously employed assistants, but here most of the works are from his own hand. And in them all, in the very paint itself, there is the drawing.

'Rubens: The Master in the Making' is at the National Gallery, London WC2, until January 15, sponsored by Shell. Tel 020 7747 2885