Unilateral naris occlusion in rats on postnatal Day 1 results in dramatic decreases in the size of specific olfactory bulb cell populations when pups are examined 30 days later (Frazier and Brunjes: J. Comp. Neurol. 269: 355-370, '88). The observed reductions must result from alterations in cell proliferation and/or survival, alternatives examined in the present study. During early postnatal development, most cells destined for the bulb are produced in regions caudal to the structure and migrate to the bulb in the massive rostral migratory stream. The dynamics of the stream were examined in both normal rats and pups with a single naris closed on Day 1. 3H-thymidine injections were made on postnatal Days 2, 5, 10, 20, and 30. Groups of pups were killed 2 hours later to assess patterns of proliferation and 24 hours later to gauge initial stages of migration. A gradient of labeled cells was observed in the stream, with higher levels occurring at more caudal locations. The supply of cells to the bulb peaked on Day 5 and was still substantial as late as Day 30. The deprivation procedure did not affect patterns of cell labeling at any stage tested, indicating the procedure does not affect early cellular proliferation. A third group of pups was examined 30 days after thymidine injection to assess both time of cell origin and survival rates. Dark granule cells and glia in the granule cell layer were produced at a consistent rate until Day 20 with cells added during the period evenly spread throughout the layer. Light granule and periglomerular cell production decreased dramatically after P5. Thirty days after injections on P2, fewer labeled dark granule cells and their associated glia were found in deprived bulbs, indicating that enhanced cell death plays a major role in the deprivation-induced decrease in cell number.