An in vitro method for assessing pigment aggregation in leucophores is described, in which isolated scales of the medaka, Oryzias latipes, were utilized. Theophylline produced reversible pigment dispersion in the leucophores. The pigment dispersion could not be inhibited by alpha and/or beta adrenergic blocking agents. Catecholamines, noradrenaline, adrenaline and isoproterenol, induced pigment dispersion in the leucophores which assumed a moderate pigment dispersion under the influence of theophylline. Dibenamine could not modify the pigment-dispersing effect of the catecholamines in the presence of theophylline. Whereas propranolol reversed the response of leucophores to the catecholamines ; the leucophores responded with a rapid pigment aggregation in response to the amines. Agonistic ranking experiments showed that the relative effectiveness of the amines in causing pigment aggregation was : adrenaline>noradrenaline>isoproterenol. These catecholamine-induced pigment aggregation responses were inhibited by dibenamine. These results indicate that the leucophores of Oryzias latipes possess alpha adrenoceptors, which mediate the pigment aggregation.