Among opiatelike peptides, stimulatory as well as inhibitory effects are encountered on adenylate cyclase activity. These actions are dependent not only on the investigated brain region but also on the applied peptide. Met-enkephalin stimulates adenylate cyclase activity in the rat brain stem (D-Met2, Pro5)-enkephalinamide and beta-endorphin inhibited it, whereas all three peptides inhibited the activity of cortex. Naloxone antagonized the effects of the applied peptides in the presence of sodium chloride.