A series of N-acyl derivatives of tyramine, tryptamine, and serotonin were synthesized and tested on anti-melanogenic activity. The serotonin derivatives such as N-caffeoylserotonin (3) and N-protocatechuoylserotonin (9) were inhibitory to tyrosinase from mouse B16 and human HMV-II melanoma cells, while the corresponding derivatives of tryptamine and 5-methoxytryptamine were almost inactive or less active than the serotonin derivatives. The inhibitory activity of the serotonin derivatives increased with increasing number of phenolic hydroxyl groups in the acyl moiety. Melanin formation in the culture of B16 cells was suppressed by 3 and 9 with no cytotoxicity in the concentration range tested (IC(50)=15, 3 and 111muM for 3, 9, and kojic acid, respectively). Thus the N-acylserotonin derivatives having a dihydroxyphenyl group are potential anti-melanogenic agents. Their inhibition of tyrosinase is primarily performed through the 5-hydroxyindole moiety and further strengthened by the phenolic hydroxyl groups in the acyl moiety.