Light and light-dark cycles have a significant effect in regulating the reproductive cycles o f some mammals .' During long nights and short days or after blinding, gonadal atrophy will occu r in the Syrian golden hamster, Mesocricetus auratus. This phenomenon has been shown to be mediated by stimulation of the pineal gland . Moreover, the light-dark cycle is believed to have a similar endocrine effect in human beings . For example, in northern Finland most conceptions occur during the summer . 2 A widely cited piece of evidence for the presence of a pineal effect on the fertility of human being s is the study of Elden . 3 In 1969, he reports only one woman blind since birth became pregnan t and delivered a normal child in the state of Wash ington. This woman had no light perception . On the basis of the birth rate and the blind population, there should have been 120 such women . In the United States, only six pregnancies in blin d women were reported, when over 1000 shoul d have been expected . From this data and other anecdotal evidence, Elden concluded that blindness caused infertility in human beings . However, Elden did not take into account the degree of light perception of all the subjects in hi s study, an important consideration, since the pine al is extremely sensitive to light . As little as 0. 5 p W/sq cm of full-spectrum white light can inhib it the usual dark-time rise in pineal N-acetyltransferase activity . Consequently, only those women without light perception should show a