Mammalian endothelins (ETs) -1, -2 and -3 effectively aggregated pigmentary organelles in xanthophores of the medaka, Oryzias latipes. IRL 1620 and sarafotoxin S6c, both being selective agonists of the mammalian ET_B receptor, were also found to aggregate xanthosomes in those cells. Quantitative studies on the action of ET-1, ET-3 and IRL 1620 on xanthophores indicated that the responses were concentration-dependent, and that they may act directly on xanthophores, because denervated cells responded to the peptides quite similarly. Blockers of some receptors known to mediate motile activity of chromatophores were ineffective in inhibiting the action of ETs. In addition, other blockers of mammalian ET_A receptors (BQ-123 and TTA-386) or ET_B receptors (BQ-788) were also disclosed to be ineffective in interfering with the action of ET. Therefore, ETs may act through the mediation of ET receptors on the medaka xanthophores that do not resemble mammalian ET receptors pharmacologically. Thus, along with their effects on other kinds of chromatophores, ETs may take part in the delicate and exquisite control of integumentary hues and patterns.