In a previous paper it was demonstrated that the mammalian retina is able to synthesize biologically active indoles. In order to investigate the effects of the environmental lighting on the retinal hydroxyindole—O—methyl transferase (HIOMT) activity, the following experiments were performed. Adult male rats were maintained: a) under a lighting schedule of 14 hr light/10 hr darkness; b) in constant light for 4 days; and c) in constant darkness for 4 days. Groups of rats were sacrificed at different times and retinal and pineal HIOMT activities determined. Both tissues presented a circadian rhythm under normal lighting conditions. The highest ocular HIOMT activity was observed during the light period, in opposition to the pineal gland, in which HIOMT was maximum in darkness. Exposure of rats to continuous light disrupted both retinal and pineal HIOMT rhythms. Rats exposed to constant darkness still presented significant cyclic changes in retinal and pineal melatonin synthesizing activities but a timing shift of the ocular HIOMT was observed, the maximum coinciding with that of the pineal gland. The present results demonstrate that an endogenous rhythm is present both in the retinal and pineal HIOMT activities of the rat. (Endocrinology91: 423,1972)