Female albino rats exposed to constant light are anovulatory and show an extensive loss of retinal photoreceptors but resume regular ovarian cycles when returned to light-dark conditions. If returned to constant light such animals develop persistent vaginal cornification at the same rate as animals with intact retinas exposed to constant light for the first time. Pigmented rats exposed to constant light may become anovulatory without any retinal changes at the light microscopic level. Possible mechanisms involved in the light-induced blockade of ovulation in the female rat are discussed.