Adult female Syrian hamsters were exposed to 10 or 14 hours of illumination daily. Animals on 14 hours of light per day showed consistent 4-day ovulatory cycles, but exposure to 10 hours of illumination daily resulted in acyclicity after about 6 weeks. The acyclic state was accompanied by large diurnal fluctuations in serum gonadotropin concentrations. These fluctuations may be responsible for the inhibition of estrous cyclicity; alternatively, they may be correlated with, but not causal to, the inhibition of estrous cycles.