Serotonin N-acetyltransferase (AA-NAT; arylalkylamine N-acetyltransferase; EC 2.3.1.87) is the penultimate enzyme in melatonin synthesis and large changes in the activity of this enzyme appear to regulate the rhythm in melatonin synthesis. Recent advances have made it possible to study the mRNA encoding chicken AA-NAT, which has only been detected in the retina and pineal gland. Within the retina, AA-NAT mRNA is expressed primarily in photoreceptors. The levels of chicken retinal AA-NAT mRNA and activity exhibit 24-hour rhythms with peaks at night. These rhythms appear to reflect circadian clock control of AA-NAT mRNA abundance and independent effects of light and darkness on both mRNA levels and enzyme activity. The effects of darkness and light may occur through alterations in cAMP-dependent protein phosphorylation, which increases AA-NAT activity in photoreceptor cell cultures. The cAMP-dependent increase of AA-NAT enzyme activity reflects, at least in part, increased mRNA levels and inhibition of enzyme inactivation by a posttranslational mechanism. This review discusses a hypothetical model for the cellular and molecular regulation of AA-NAT activity by circadian oscillators and light in chicken retinal photoreceptor cells.