To assess the implication of age at the time of exposure to chronic irradiation, rats were exposed to constant tritium (HTO) activities of 10 microcuries/ml of body water for 42 days beginning either on the first 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 first day of pregnancy or birth was evident upon examination at 49 days of age. Females exposed from the first 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.