OBJECTIVE The authors examined the effects of both light and circadian phase on the synthetic activity of the dopamine producing enzyme, tyrosine hydroxylase (TH), in the retina of the Midas cichlid (Cichlasoma citrinellum). METHODS TH activity was assayed in the retina-retinal pigment epithelium (RPE) complex of fish entrained to cyclic light regimens. The animals were killed throughout cyclic light and continuous dark cycles and after light exposure following short-term dark adaptation to assess the effects of both diurnal and circadian phase and light exposure on TH activity. Variations in retina-RPE TH activity were compared with cone retinomotor position. RESULTS The authors report that TH activity was influenced both by light and by circadian phase. In both cyclic light and continuous darkness, TH activity fluctuated in cyclic fashion, increasing to peak values at night, declining through dawn, and reaching minimal values at midday. In continuous dark-adapted retinae, the peak activity at night was approximately sixfold greater than the minimum activity during the day. In cyclic light-adapted retinae at midday, the TH activity was 2.4-fold higher than that determined at the same time point in continuous dark-adapted retinae. Light onset enhanced the TH activity in previously dark-adapted animals by 2.2-fold at midday and 1.7-fold at dusk. Thus, retinal TH activity exhibits fluctuations of approximately sixfold in response to circadian signals and is stimulated approximately twofold by light. CONCLUSIONS These observations indicate that dopamine production in the retina is regulated both by light and by circadian phase and that there are cyclic changes in dopamine production during the 24-hr cycle both in normal light and dark cycles and under conditions of constant darkness.