The mRNAs encoding three enzymes of the melatonin synthesis pathway (tryptophan hydroxylase (TPH), arylalkylamine-N-acetyltransferase (AANAT) and hydroxyindole-O-methyl-transferase (HIOMT)) are expressed with a day/night rhythm in the chicken pineal gland and retina. TPH and AANAT mRNA levels reach their peak at night. HIOMT mRNA levels peak at night in the retina, but during the day in the pineal gland. In this tissue, the rhythm of TPH, AANAT and HIOMT mRNA levels persisted in constant darkness (DD), both in vivo and in vitro, indicating that the three genes are controlled by the circadian oscillator of the chicken pineal. In the retina, the rhythms of TPH and AANAT mRNA levels also persisted in DD in vivo, suggesting that they are driven by a circadian oscillator. In contrast, the rhythm of HIOMT mRNA in the retina appeared to be controlled only by light. The clones of chicken AANAT and HIOMT genes that we have isolated should help us to understand the molecular mechanisms of: 1) their transcriptional regulation by circadian oscillators and by light; 2) their tissue-specific expression in the pineal gland and the retina.