Ergonovine maleate inhibits melanophore stimulating hormone (MSH) secretion from the incubated frog, rat, and mouse pituitary gland. This ergot alkaloid is partially effective at a concentration as low as M. The inhibitory effect on hormone release is irreversible under the experimental conditions studied. Ergonovine inhibition of MSH secretion is blocked by Dibenamine and chlorpromazine as is dopamine inhibition of MSH secretion. At higher concentrations chlorpromazine displays an intrinsic ability to inhibit MSH secretion. Similarities between the cellular mechanisms controlling MSH and prolactin secretion will be discussed.