The responses of melanophores in isolated scales of Oryzias latipes to beta−phenylethylamine were investigated under a microscope．Beta phenylethyl− amine（10^<-6>-1O^<-3>M）induced concentration of the pigment within innervated melanophores，but did not do within denervated ones. When beta-phenylethyl- amine was locally applied on a scale with innervated melanophores，only those in the applied area responded in clear contrast to the remaining ones which remained in a condition of full dispersion． These results may be interpreted to indicate that the drug may acts upon the concentrating nerve endings to cause a release of an active neurohumour．The failure of the direct action on melanophores may be attributable to the absence of the phenolic OH group on the rmg in the structure of the drug The m．elanophores treated． with the drug maintained a concentrating state for long time, even after the test solution was replaced with physiological soiution．