The response of prolactin (PRL) and that of the pituitary-testicular axis to inhibitory photoperiods were compared in immature and young adult hamsters. In the first experiment, 14-day-old hamsters were transferred from 14L:10D to 12L:12D or 8L:16D. At 21, 28, 35, and 49 days of age, body weights, testes weights, and plasma LH were similar in all groups. In contrast, the levels of PRL were lower in animals on 8L:16D or 12L:12D than in those on 14L:10D and 49 days of age. In a second experiment, 80-day-old hamsters were transferred from 14L:10D to either 12L:12D or 8L:16D. At 5-wk intervals, they were weighed and bled by cardiocentesis and the length of the testes was measured. Photoperiods of 12L:12D and 8L:16D induced testicular regression, which was complete by 15 wk. Thereafter, augmented levels of FSH preceded spontaneous recrudescence of the testes in animals on both photoperiods. Serum PRL levels were significantly depressed within 5 wk after transfer to either 8L:16D or 12L:12D. After 15 wk as the testes were increasing in size in both groups, circulating PRL levels increased in animals exposed to 8L:16D, whereas in animals exposed to 12L:12D, PRL remained at or below the sensitivity of the assay. In a final experiment, on the day before parturition, pregnant hamsters were transferred to either 6L:18D, 6L:30D, 6L:42D, or 6L:54D or were held on 14L:10D. There were no differences in testicular weights, plasma LH, or plasma thyroxine (T4) of the pups from these dams at 21, 36, or 48 days of age.(ABSTRACT TRUNCATED AT 250 WORDS)