The effect of reserpine on hypothalamo-pituitary-adrenocortical (HPA) function in the rat was investigated by the use of direct and indirect indices of pituitary adrenocorticotrophic activity. 2. Administration of a single dose of the drug induced prolonged hypersecretion of corticotrophin (ACTH). 3. Corticotrophin release in response to the drug no longer occurred after repeated daily injections, indicating that some form of 'adaptation' occurred. 4. The increase in HPA activity normally caused by exposure to cold was prevented by reserpine once 'adaptation' to the drug had been produced. 5. Inhibition of stress-induced ACTH release was due neither to depletion of pituitary stores of the hormone, nor to a corticosteroid feedback effect.
