The mechanism of melanosome-aggregating action of dopamine on fish melanophores was investigated, using isolated scales of Oryzias latipes. Dibenamine blocked the melanosome aggregation response of the melanophores to norepinephrine and to dopamine. Haloperidol inhibited the response to dopamine in both the innervated and denervated melanophores . The inhibition was pronounced in the denervated melanophores. Haloperidol accelerated the melanosome aggregating action of norepinephrine in the innervated melanophores, whereas in the denervated ones the drug inhibited the norepinephrine action . From these results, the following conclusions were suggested : The melanosome aggregation response induced by dopamine is not mediated through dopamine-specific receptors, but through alpha adrenergic receptors on the melanophores . Dopamire may act on the presynaptic endings to induce the release of the transmitter. Haloperidol also interacts with alpha adrenergic receptors of the melanophores.