Systemic injections of 5-hydroxytryptophan (5-HTP), the precursor to serotonin, stimulates melanin dispersion within dermal melanophores of red-spotted newts (Notophthalmus viridescens). Injections of para-Chlorophenylalanine (PCPA), an inhibitor of serotonin synthesis, inhibited melanin dispersion, and hence darkening of the skin, when newts were transferred to a dark background. The results indicate a role for serotoninergic activity in the background adaptation response in this amphibian.