Serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AA-NAT, HGMW-approved symbol AANAT; EC 2.3.1.87) is the penultimate enzyme in melatonin synthesis and controls the night/day rhythm in melatonin production in the vertebrate pineal gland. We have found that the human AA-NAT gene spans approximately 2.5 kb, contains four exons, and is located at chromosome 17q25. The open reading frame encodes a 23.2-kDa protein that is approximately 80% identical to sheep and rat AA-NAT. The AA-NAT transcript (approximately 1 kb) is highly abundant in the pineal gland and is expressed at lower levels in the retina and in the Y79 retinoblastoma cell line. AA-NAT mRNA is also detectable at low levels in several brain regions and the pituitary gland, but not in several peripheral tissues examined. Brain and pituitary AA-NAT could modulate serotonin-dependent aspects of human behavior and pituitary function.