To assess the implication of age at the time of exposure to chronic irradiation, rats were exposed to constant tritium (HTO) activities of 10 .mu.Ci/ml of body water for 42 days beginning either on the 1st day of pregnancy or at birth, or at 42 days or 74 days of age. This activity level provided a calculated whole-body dose rate of 3 rad/day. The indicators of radiobiologic damage employed were reproductive, endocrine and neurochemical parameters and were assessed at 49, 120 or 300 days of age. A significant reduction in the testes weight and sperm content in rats exposed either from the 1st day of pregnancy or birth was evident upon examination at 49 days of age. Females exposed from the 1st day of pregnancy had a significant reduction in their F2 litter size and an increase in the number of resorbed embryos. Analysis for norepinephrine (NE) and dopamine (DA) concentration in the brain and follicle-stimulating hormone (FSH) concentration in the pituitary was performed at 49 and 300 days. Rats exposed from conception had increases in NE and DA and a decreased FSH concentration when examined at 49 days of age. When examined at 300 days of age all groups had decreased NE and DA concentrations. At the same time, all treatment groups showed increased FSH concentration with the groups exposed later in life showing the greatest increase. From the parameters examined all age gorups showed some effects due to the HTO exposure, but the group most sensitive to the effects of chronic irradiation was the group exposed from the 1st day of pregnancy.