Unilateral olfactory deprivation on Day 1 results in large reduction in olfactory bulb size when animals are examined in the juvenile period. The present study explored 2 issues. First, the development of changes in laminar volume was measured in order to determine when differences in olfactory bulb size emerge. Rat pups underwent surgical closure of one naris on Day 1 and laminar sizes were examined on either day 4, 8, or 12. Reliable differences emerged between "deprived" and control bulbs by Day 12. Second, to determine the length of the period of susceptibility to changes in sensory input, pups were deprived on either Day 10 or 20 and reared for thirty days. Deprivation at both ages reduced bulb size, although treatment at the later age resulted in less severe alterations.