Several variables were examined at nine time periods over a 24-hour period in rats following prolonged exposure to constant light (LL), compared with animals housed in an alternating light-dark (LD) environment. The data indicated: LL animals had lower ovarian weights and increased uterine weights, which were not dependent on the time of day of autopsy; pituitary LH content was lower in LL animals, and dependent on time of day only in LD rats; there were no significant differences in serum LH concentrations between the LL and LD animals, nor among times of blood collection; serum FSH concentrations varied significantly with time, but were not different between LL and LD animals. It is suggested that, in the absence of cyclic surges of LH, low tonic levels of serum LH can support estrogen secretion, resulting in higher uterine weight, and that fluctuating but low serum FSH levels may help maintain the follicular stimulation observed in LL animals.