Young rats exposed to an odor while receiving reinforcing stimulation come to approach that odor upon subsequent presentation. In addition, such pups have increased 14C-2-deoxyglucose (2DG) uptake within focal areas of the glomerular layer in response to that odor, compared to control animals experiencing the odor for the first time. In this study, the morphology of the glomerular areas underlying these 2DG foci was examined to determine whether early olfactory learning imposed local structural changes that could produce the enhanced 2DG uptake. Alternate sections either were processed with a silver and a Nissl stain to label both cell bodies and their processes or were histochemically treated for the mitochondrial enzymes cytochrome oxidase (CO) or succinic dehydrogenase (SDH) to define the glomerular core of the bulb; 2DG autoradiographs were aligned with adjacent stained sections, and regions underlying the high 2DG uptake foci were examined. In odor-familiar animals, large glomerular clusters that protruded into the external plexiform layer or the olfactory nerve layer were associated with the focal areas of increased 2DG uptake. Morphometric analysis of these regions revealed that the glomerular layer underlying the foci of high 2DG uptake was 30% wider in odor-familiar animals than comparable areas in odor-unfamiliar animals; the cross-sectional areas of individual glomeruli were 21% larger in odor-familiar animals. The foci of enhanced 2DG uptake therefore appear to be associated with groups of enlarged glomeruli. These data demonstrate that early olfactory learning influences the morphology of the olfactory bulb.