This study was performed to investigate the reactivity of melatonin towards various chemical reagents to produce new derivatives of melatonin containing extra- or fused heterocyclic systems such as the imidazoindole derivatives 3, 5, 6, the triazinoindole derivatives 8a, b, the thiadiazoloindole derivative 12, the pyridinoindole derivative 16 and the aminopyrazolomelatonin derivatives 21a, b and 22a, b. The structures of the compounds were established based on the analytical and spectral data. Agonist and antagonist potency of some melatonin derivatives in clonal Xenopus laevis melanophore cells was studied in comparison with melatonin.