Studies were conducted to determine the period of immune system development that was most sensitive to perturbation by the known immunotoxicant di-n-octyltin dichloride (DOTC). Fischer 344 rats were exposed prenatally, both pre- and postnatally, or postnatally to DOTC by oral gavage of pregnant and/or lactating females. At various ages, ranging from 3 to 16 wk of age, offspring were examined for a number of immune functions. These included body and lymphoid organ weights; lymphoproliferative responses to B- and T-cell mitogens; natural killer cell activity; and primary antibody response to sheep erythrocytes. Prenatal (10-20 of gestation), pre- and postnatal (d 11-20 of gestation and 2-11 d of age), or postnatal (2-13 d of age) oral dosing of dams with 20-50 mg/kg DOTC resulted in no consistent alteration in immune function in offspring. However, direct oral dosing of rat pups to 5-15 mg/kg DOTC, beginning at 3 d of age and then 3 times per week up to 24 d of age for a total of 10 doses, resulted in significant suppression of the lymphoproliferative response of splenocytes to a T-cell mitogen in 10-wk-old rats (i.e., 7 wk after the last exposure to DOTC). Lymphoproliferative responses returned to control levels by 12 wk of age. In comparison young adult (8 wk old) rats dosed with 10 or 20 mg/kg DOTC under an identical dosing schedule (i.e., 3 times per week for a total of 10 doses) showed no suppression in the mitogen response of splenocytes 4 wk after the last exposure to DOTC. These results suggest that direct dosing of pups during early postnatal life may be the most effective means of inducing immunosuppression with DOTC during immune system development. The results also provide evidence for the greater sensitivity of the developing immune system compared with the fully developed immune system for a known immunotoxicant.