The time course of select phagocyte and natural killer activities to short days was determined. In advance of testes regression, circulating granulocyte and monocyte cell numbers in hamsters decreased while lymphocyte numbers increased; phagocytosis and oxidative burst activity also decreased. To determine whether the pineal gland influences these innate immune cell functions, hamsters were exposed to constant light. Photoperiod control of testes weight and basal oxidative burst activity was abolished by treatment with constant light; other phagocyte activities and leukocyte proportions in circulation were not affected. The findings suggest that photoperiod and pineal gland function may regulate certain innate immune activities.