The present study describes the ability of 315 nM okadaic acid to induce melanosome dispersion in culturedXenopus laevismelanophores. This effect of okadaic acid is similar to that of a‐melanocyte stimulating hormone (MSH) and can be reversed by melatonin treatment; it indicates that a member of the protein‐phosphatase 1 or 2A families must be active for maintenance of the aggregated state. Higher concentrations of okadaic acid (1 μM) attenuate the response ofXenopusmelanophores to melatonin leading to the hypothesis that melatonin action is mediated by the calcium/calmodulin activated phosphatase 2B. This hypothesis seems unlikely, however, since the calcium/calmodulin inhibitors TFP and W7 do not prevent melatonin‐induced pigment aggregation, but instead induce aggregation on the
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