In the Agouti mice C3H A" the coat color of the dorsum changes from birth to maturity. In young animals the dorsal tegument is yellow whereas in adult mice it shifts to dark gray. The administration of bromoergocryptine, a dopamine agonist, as a single injection in a beeswax pellet prevented the color change resulting in the persistence of the immature pattern. After plucking an area of the dorsum of an adult animal, the regrown hair was dark; however, when bromoergocryptine was administered at the time of plucking the new hair was not dark but yellow. When MSH was injected in adult animals previously treated with the drug, the ‘bleaching’ effect of bromoergocryptine was abolished, suggesting that this substance did not act directly upon melanin synthesis. In bromoergocryptine-treated mice the MSH content of the pars intermedia was decreased and the ultrastructure of this lobe contained cells with signs of hypoactivity. These two observations suggested that the effect of the drug on the coat color might be ascribed to inhibition of the secretion of MSH.