Abstract—Recent evidence suggests that immune function varies substantially on seasonal basis. The primary cue to perceive seasonal change is change in photoperiod. The purpose of the present experiment was to study the role of photoperiodic manipulation on the nitric oxide production by peripheral blood leucocytes in the fresh-water snake, Natrix piscator . To study effect of photoperiod, animals were subjected to 24 hour continuous light and continuous dark for 30 days. Animals kept in natural day length served as control. At termination of experiments, animals were sacrificed, and blood was collected. Leucocytes were isolated and were incubated for 24 hours and nitric oxide production was measured by measuring the nitrite concentration. Nitrite production was significantly decreased to the cultures obtained from the animals kept under continuous light while nitrite concentration was increased in animals kept under continuous dark, when compared with the animals kept under natural day length. The possible role of increased melatonin synthesis in dark was suggested to increase the nitric oxide production.