Immature female rats, raised in either cyclic light (LD; 14 h light/10 h dark) or constant light (LL) were divided into 4 groups and given daily injections of estradiol benzoate (5, 10, or 25 ng) in 0.1 ml sesame oil starting at 24 days of age. Control animals received oil only. Injections were continued until vaginal opening (V.O.). Half of each group of animals was killed and dissected 1 day after V.O. Results indicate that both LL exposure and EB treatment accelerated sexual maturation (as indicated by the day of V.O.) with a significant interaction between the 2 treatments. The greatest difference in age at V.O. between LD and LL rats was observed at a dose of 10 ng EB/day. Body weights and uterine weights decreased with higher doses of EB, apparently reflecting the younger age at the time of V.O. No significant differences in ovarian weights were observed. Results are interpreted to indicate that increased exposure to light may change the sensitivity of the hypothalamic-pituitary axis to the positive feedback of estrogen.