The possible involvement of catecholamines in the regulation of serotonin N-acetyltransferase (NAT) activity in retina of the African clawed frog was investigated using an in vitro eye cup preparation. Dopamine (10 microM) and norepinephrine (50 microM) had no significant effect on NAT activity of eye cups incubated in the light. However, dopamine inhibited the increase of retinal NAT activity that occurs in eye cups incubated in darkness; the ED50 for dopamine was 0.3 microM. The effect of dopamine on NAT activity was mimicked by the dopamine receptor agonists apomorphine and bromocriptine, but not by agonists of alpha 1-, alpha 2- or beta-adrenergic receptors. Dopamine-mediated inhibition of NAT activity was antagonized by spiroperidol and by alpha-flupenthixol, but not by beta-flupenthixol, phentolamine or timolol. Benztropine, an inhibitor of dopamine reuptake, also decreased NAT activity in eye cups incubated in the dark. The inhibitory effect of benztropine was antagonized by spiroperidol, suggesting that it was mediated by an increase in the extracellular concentration of endogenous dopamine. These studies indicate that the regulation of NAT activity in the retina is subject to modulation by a dopamine receptor-mediated mechanism and suggest that dopamine may play a role in the inhibition of NAT activity by light.