Estradiol-17 beta (estradiol) inhibition of tonic (basal, as opposed to cyclic or preovulatory) serum luteinizing hormone (LH) and follicle-stimulating hormone (FSH) was evaluated in ewes subjected to long or short photoperiods. Nine mature Colombia ewes were ovariectomized early in the breeding season and implanted with either Silastic capsules containing approximately .12 g of estradiol (n = 4) or with empty capsules (n = 5). Ewes were housed indoors for 43 wk under controlled lighting: 17 wk of short days (8.5 h light: 15.5 h dark), 12 wk of long days (15.5 h light: 8.5 h dark) and a second 14 wk of short days. Five intact ewes penned with a vasectomized marker ram were used to evaluate estrus under these photoperiodic conditions. Estrous cycles in intact ewes ceased approximately 10 wk after onset of the long photoperiod and resumed approximately 11 wk after return to short days. In ovariectomized ewes, estradiol dramatically depressed FSH as well as LH during the long photoperiod and, to a lesser degree, depressed FSH, but not LH, during the short photoperiod. These data support the hypothesis that sensitivity of the tonic gonadotropin regulatory mechanism to estradiol inhibition changes with photoperiod and, thereby, modulates the onset and duration of the breeding and anestrous seasons in ewes. A model accounting for seasonal changes in the inhibitory action of estradiol is proposed.