Using leucophores in isolated scales of a fresh-water teleost, Oryzias latipes, experirnents were designed to characterize the nature of adrenergic receptors mediating the pigment-dispersing response. Adrenaline, Noradrenaline and isoproterenol caused dispersion of the pigment within both innervated and denervated leucophores, indicating that these amines act directly on the cells. The order of potency of these catecholamines for inducing the pigment dispersion within the leucophores was isoproterenol> noradrenaline> adrenaline. Pretreatment with dibenamine or ergotamine, alpha adrenergic blocking agents, did not affect the responses of leucophores to these agonists. On the other hand, propranolol or dichloroisoproterenol, beta adrenergic blocking agents, inhibited the pigment-dispersing effect of these amines. From these results, it was concluded that the adrenergic receptors mediating pigment dispersion within the leucophores are of beta nature.