Summary. Male voles were raised from birth to 100 days of age in photoperiods of 16L:8D or 6L:18D. In the long photoperiod testes increased in size between 15 and 80 days of age, and there was an increase in seminal vesicle weight from 60 days of age. Spermatozoa were present in the testes at 60 days of age. In the short photoperiod testicular growth did not begin until 50 days of age with the seminal vesicles beginning to increase at 80 days of age. Spermatozoa were present in the testes at 100 days of age. Pituitary secretion in vitro of LH and FSH in response to 1 pmol GnRH, as well as hypothalamic GnRH content, rose to peaks at 50 and 80 days of age respectively in animals exposed to long photoperiods. There was no change in pituitary secretion of FSH in response to GnRH stimulation in animals from the short photoperiod. However, pituitary release of LH in response to 1 pmol GnRH rose to a peak at 80 days of age. Hypothalamic GnRH content rose to a peak at 50 days of age and then declined. The relationship between the hypothalamic GnRH and the sensitivity of the pituitary to GnRH stimulation is compatible with the idea that GnRH can mediate its own receptor numbers.