Circulating concentrations of prolactin were monitored for 3 yr in intact ewes kept either outdoors or indoors in a fixed equatorial photoperiod (12L:12D) and restricted range of environmental temperatures. Prolactin data were analyzed by spectral analysis. In all ewes kept outdoors, concentrations of prolactin showed robust circannual rhythms with a single predominant period of 359 days. In ewes kept indoors, the range of significant periods varied from 35 to 532 days. Although all ewes kept indoors showed a significant rhythm with a period of 354 days, this clearly was not the predominant period in all. The amplitude of the rhythm in ewes kept indoors was significantly lower (p less than 0.01) than that of ewes kept outdoors. Although the annual rhythm of circulating prolactin typical of ewes kept outdoors was significantly compromised in animals kept under a constant 12L:12D photoperiod and restricted environmental temperature range, there was evidence of an endogenous circannual rhythm.