Clinique Endocrinologique, C.H.U. La Timone, Marseille 13385, France (Received 23 September 1975) Melanocyte-stimulating hormone (MSH) and adrenocorticotrophin (ACTH) release vary independently in the rat following corticosteroid administration or adrenalectomy as shown by bioassay (Kastin, Schally, Viosca & Miller, 1969; Dunn, Kastin, Carrillo & Schally, 1972; Thody & Hinks, 1973) or radioimmunoassay (Usategui, Oliver, Vaudry, Lombardi, Mourre, Rozenberg & Vague, 1975; Usategui, Oliver, Vaudry, Lombardi, Rozenberg & Mourre, 1976) methods. In an attempt to define more clearly the effect of corticosteroids on the release of α-MSH and ACTH, plasma α-MSH and ACTH have been measured after stimulation by haloperidol injection or ether stress in normal as well as in corticosteroid-treated male rats. Immunoreactive α-MSH has been used as an index of MSH activity because α-MSH is thought to be the major MSH in the rat (Baker, 1973; Scott, Lowry, Ratcliffe, Rees & Landon, 1974; Thody, Penny & Clark, 1975). Adult male