Cytochrome oxidase staining selectively highlights the synapse-rich neuropil of olfactory bulb glomeruli. Detailed morphometric enumeration of glomeruli in cytochrome oxidase-stained sections from postnatally developing rats reveals that the entire adult population of glomeruli (2400/bulb) forms early in life, the process being complete by days 3-5 postnatal. Newborn's glomeruli range in diameter from a mean of about 50 microns to a maximum of about 70 microns and undergo a 3-fold increase in diameter and a 20-fold increase in volume during days 1-50 postnatal. The presence of the entire adult's glomeruli in the neonate calls for a serious revision of some current views on glomerular development and, given the critical roles of glomeruli as modules, the findings bear important implications for the organization of olfactory system and early development of olfactory functions.