We have cloned the tryptophan hydroxylase cDNA from hamster pituitary and demonstrated its expression in the skin, melanotic and amelanotic melanomas, spleen, heart, and the eye. We further demonstrated that skin, melanomas, spleen, pituitary, and eye but not heart expressed arylalkylamine N-acetyltransferase mRNA. The cutaneous expression of the arylalkylamine N-acetyltransferase gene was accompanied by enzymatic activity for the conversion of serotonin and tryptamine to N-acetylserotonin and N-acetyltryptamine, respectively. There was marked regional variation in the serotonin N-acetyltransferase activity, which was higher in ear skin than in corpus skin, and was lower in melanomas than in normal skin. Serotonin N-acetyltransferase activity was significantly inhibited by Cole bisubstrate at low concentration (</= 1 micro m); this evidence in conjunction with arylalkylamine N-acetyltransferase mRNA expression implies an involvement of arylalkylamine N-acetyltransferase in serotonin metabolism in the skin. We also documented both the in vitro transformation of serotonin to N-acetylserotonin using liquid chromatography/mass spectrometry and the generation/storage of N-acetylserotonin in cultured melanoma cells. Thus, we have uncovered a cutaneous pathway displaying capabilities for serotonin biosynthesis and/or its metabolism to N-acetylserotonin in rodent skin. As serotonin has powerful vasodilator, immunomodulator, and growth factor actions, this pathway could be involved in skin physiology and/or pathology.