The ['4C]2-deoxy-D-glucose (2DG) technique was employed to study the ontogeny of functional organization in the olfactory bulbs of rats from birth to 21 days postnatal. These observations were correlated with the histological maturation of the olfactory bulb laminae. In O-day pups (within 12 hr of parturition), foci of increased metabolic activity were elicited in the olfactory bulb by the odor of amyl acetate. The foci, generally poorly defined, were localized over the glomerular layer, which, in histological sections, was poorly differentiated. In suckling pups injected with 2DG, a single focus characteristically was observed in the main olfactory bulb near the medial border of the accessory olfactory bulb. In this region, a well differentiated complex of glomeruli was observed in the histological sections.During the ensuing 21 days postnatal, the focal patterns of 2DG uptake in animals exposed to amyl acetate odor progressed through a series of changes, each of which could be related to parallel stages of development in the histology of the bulb. Particularly striking were the establishment of sharply defined 2DG foci and the formation of distinct individual olfactory glomeruli by the end of the 1st week. Generally, by 15 days postnatal, the adult patterns of 2DG uptake and histological lamination were established.The modified glomerular complex also exhibited focal 2DG uptake beginning on day 0, but these were better defined than those associated with the glomeruli of the main olfactory bulb. This observation correlated with the more advanced histological differentiation of this specialized region. These data suggest that the modified glomerular complex may mature earlier than other regions of the olfactory bulb. This earlier maturation may reflect the importance of the modified glomerular region for processing odor cues relevant to suckling behavior in neonatal rats.Taken together, these results provide evidence for several successive changes in the functional organization of the developing olfactory system of neonatal rats.The neural substrates of olfactory function in the neonatal rat are not well understood. Behavioral studies generally agree upon the critical role of olfaction during the perinatal period of development (Blass and Teicher,