A characterization of nitric oxide synthase (NOS) in the golden hamster retina was performed. Enzymatic activity was partially Ca2+ and calmodulin-dependent, required NADPH, and was inhibited by L-arginine analogs. Retinal NOS activity was higher at midnight than at midday. When the hamster were placed under constant darkness for 24 or 48 h, and killed at equivalent time points, representing subjective day and subjective night respectively, the differences in NOS activity disappeared. One hour of darkness during the day increased, while the same period of light during the night decreased, retinal NOS activity. From these results, it might be presumed that hamster retinal NOS activity is regulated by the photic stimulus.