In 44 patients with acute renal failure serum prolactin levels were estimated using chloropromazine as a stimulant and alfa-bromocriptine as an inhibitory agent. Significantly elevated basal serum prolactin levels were found both in oliguric and polyuric patients with ARF. Intramuscular (i.m.) administration of chloropromazine markedly increased serum prolactin concentrations. In patients with ARF the apparent half life of prolactin, calculated from the prolactin curve obtained after alfa-bromocriptine administration, was moderately longer than in normals. From the results obtained the following conclusions were drawn: 1. Hyperprolactinemia of patients with ARF seems to be due predominantly to hypersecretion of this hormone and, to a lesser degree, to impair renal biodegradation. 2. In patients with ARF prolactin secretion is hyperresponsive to intramuscularly administered chloropromazine.