Ouabain, a sodium- and potassium-dependent ATPase inhibitor, induced irreversible pigment aggregation in the innervated melanophores of Oryzias latipes, but in the denervated ones it failed to cause pigment aggregation or pigment dispersion, suggesting that this drug acts on the presynaptic nervous elements to promote spontaneous release of the transmitter which produces pigment aggregation within melanophores. Since ouabain did not affect responsiveness of the melanophores to directly acting substances, it was concluded that a Na-K ATPase did not participate directly in pigment movements. The effect of ouabain on the nervous elements was antagonized by excess potassium ions in the external medium.