Dramatic alterations occur in the developing olfactory bulb when air flow is reduced through one-half of the nasal cavity. Naris closure on the day after the day of birth (P1) in rats, for example, results in reduced cell survival in the ipsilateral bulb by P20 and a substantial (25%) decrease in bulb size by P30. Almost immediate changes in protein synthesis and cell metabolism are also observed, and one prevalent theory suggests that these changes may be important in specifying which cells are subsequently eliminated. In the present study we used a reversible technique for unilateral naris closure to examine the sensitive period for the effects of olfactory deprivation on bulb size and cell survival. This technique involves the insertion of removable plugs into a rat pup's external naris. We occluded the naris for increasing periods of time (P1-P10, P1-P15, or P1-P20), reared all animals to P30, and measured volumes of bulb laminae. In addition, we examined the duration of naris closure needed to affect cell survival by injecting animals with the thymidine analogue bromodeoxyuridine to label cells born soon after the onset of olfactory deprivation. Results indicate that relatively long periods of naris occlusion (P1-P15 or longer) are required to produce a substantial reduction in experimental bulb size. Cell survival was decreased following olfactory deprivation from P1 to P10, but not after deprivation from P1 to P3. These data support the hypothesis that changes that occur within 48 h of naris closure are not sufficient to affect cell survival.