During recent experiments in our laboratories it was noted that rats pharmacologically blocked with dexamethasone, morphine and sodium pentobarbitone showed increased plasma levels of melanocyte-stimulating hormone (MSH) and suppressed plasma levels of corticosterone. Since there exists increasing evidence that MSH has an extra-pigmentary function in mammals (DeWied, Bohus & Greven, 1968; Kastin, Kullander, Borglin, Dahlberg, Dyster-Aas, Krakau, Ingvar, Miller, Bowers & Schally, 1968; Kastin, Miller, Gonzalez-Barcena, Hawley, Dyster-Aas, Schally, DeParra & Velasco, 1971) and since there appears to be some discrepancy regarding the relationship of MSH and corticotrophin (ACTH) to various stimuli (Abe, Nicholson, Liddle, Orth & Island, 1969; Kastin, Schally, Viosca & Miller, 1969), it seemed appropriate to determine the effect of the pharmacological block — dexamethasone, morphine and sodium pentobarbitone — on the release of MSH and ACTH. Experiments were performed using adult (160–200 g) female Sprague—Dawley rats (Southern Farms) that were housed two to a cage for