We have reported that regeneration of transected axons occurred in the central olfactory pathway in rats whose lateral olfactory tracts (LOTs) were transected at early postnatal days (P). The present study was undertaken to provide quantitative data on the total number of bulbar projection neurons (mitral cells) labeled with Fluoro-Gold (FG) that was injected into the posterior olfactory cortex, a target brain region of mitral cells, 12 weeks after LOT transection. The total number of FG (+) mitral cells larger than 15 μm was measured using an optical fractionator with StereoInvestigator software. Neuron numbers of FG (+) mitral cells from normal adult rats were 49700±4300 (n = 5). Neuron numbers of regenerated FG (+) mitral cells from P2 and P7 LOT-transected adult rats were 35100± 4500 (n = 5) (71% of the normal value) and 19700±5000 (n = 5) (40% of the normal value), respectively. In our previous paper, it was revealed that all (20/20) P2 LOT-transected adult rats exhibited olfactory discriminative ability as shown by 100% correct responses. Olfactory function of P7 LOT-transected adult rats was examined by the ability to discriminate between the smell of water and cycloheximide solution after removal of the olfactory bulb contralateral to the LOT transection. Most (78% 14/18) rats showed 100% correct responses, but some (22% 4/18) rats showed 50% correct responses, indicating loss of olfaction. Neuron numbers of regenerated FG (+) mitral cells from 2 olfaction (+) rats were 23400 (47% of the normal value) and 21300 (43% of the normal value), whereas those numbers from 2 olfaction (-) rats were 13000 (26% of the normal value) and 9600 (19% of the normal value). In our previous paper, it was revealed that adult rats with undamaged healthy mitral cells of less than 22% of the unilateral normal value lacked olfactory discriminative ability in acute LOT-lesioning experiments. The present study implies that approximately 25% of unilateral bulbar projection neurons is a neuronal population essential for the preservation of olfactory function both in normal and regenerated conditions.