Many cnidarians exhibit a diurnal rhythm of expansion and contraction. Previous studies correlated this behavior to various environmental factors, including prey-catching, the photosynthetic requirement of the zooxanthellae, and tidal movement in the intertidal zone. Whether intrinsic factors affect the diurnal rhythm of expansion/contraction were not addressed. The present study investigated the effect of the photoperiod hormone melatonin (N-acetyl-5-methoxytryptamine) and its homologue on the expansion and contraction of a sea anemone. Addition of exogenous melatonin at 10 -4 M induced the expansion of the oral disk. At an exogenous concentration of 10 -3 M, protrusion of the actinopharynx was induced. Melatonin thus exerted its effect in the sea anemone in a dose-dependent manner. Addition of another indoleamine, 5-methoxytryptamine, at a concentration of 10 -4 M resulted in the thythmic contraction of the body column, resembling the respiratory rhythm of coelenteric flushing. Serotonin (5-hydroxytryptamine), however, did not induce any apparent effect at an exogenous concentration of 10 -3 M. Simultaneous addition of melatonin and 5-methoxytryptamine induced both types of responses, suggesting that their effects are not antagonistic with each other. This and their different responses suggested that the two indoleamines acted on two different sites in the sea anemone. At a concentration of 10 -4 M, melatonin was also able to modulate the effect of Antho-RF-amide in the contraction of the oral disk. Eserine, an inhibitor of melatonin deacetylation, also induced protrusion of the actinopharynx. This strongly supported the concept that melatonin was produced endogenously and probably at or near the site of action.