The effects of unilateral naris closure, for 1-6 months, on the olfactory bulbs of adult mice were investigated in 3 experiments. The bulbs on the closed, or deprived, sides were smaller in a total of 68 out of 73 mice. Experiment 1 revealed a significant overall difference in cross-sectional areas of the open and closed-side bulbs of 6-, 8- or 12-week naris closure mice. In Expt. 2, in the 1-month naris closure group the difference in weights between the open and closed-side bulbs was a marginally significant 17% (P less than 0.06), while in the 3.5-month closure group the difference was 28% (P less than 0.01). In Expt. 3, after 6 months of naris closure the disparity was similarly 27% (P less than 0.01). A comparison of the size of control bulbs at the beginning of the 6-month closure period (Day 0 controls) with control sizes at the end of the period (6-month controls), revealed no significant difference (P greater than 0.05). This indicates that the smaller size of the closed-side bulbs is attributable to atrophy rather than arrested growth during the closure period. These data provide evidence that for olfaction, unlike other sensory systems, there may be no critical period during growth and development in which deprivation must occur in order to cause gross morphological changes in the CNS sensory structures. Thus the olfactory receptor neurons of adults appear to have a unique trophic relationship with their CNS target tissue, the olfactory bulbs.