1. Adenosine and its derivatives induced dispersion of leucosomes in leucophores of the medaka, Oryzias latipes. 2. Among the purines used, 5'- N-ethylcarboxiamideadenosine was the most effective and its potency was far greater than that of adenosine, N 6- l- phenylisopropyladenosine and N 6-cyclohexyladenosine. 3. Methylxanthines inhibited the purine action competitively, but beta adrenergic antagonists and dipyridamole did not. 4. Beta adrenergic agonists and forskolin synergistically augmented the purine action, while Li + blocked it competitively. 5. The results suggest that medaka leucophores possess A 2 adenosine receptors on the cell membranes, the stimulation of which induces leucosome-dispersion response by increasing the cellular level of cyclic AMP through activation of adenylate cyclase activity.