Immature chickens were implanted with osmotic pumps filled with ACTH1-24 or pellets consisting of mixtures of cholesterol with corticosterone (0, 10, 20 or 40% by weight). Continuous infusion of ACTH1-24 (2.2 micrograms/hr:120 micrograms/kg body wt/day) caused increases in plasma concentrations of corticosterone, glucose, cholesterol, triglyceride, and uric acid during the first week, and a reduction in weight gain, an increase in the relative weights of the adrenals and liver, and a decrease in the weights of the bursa and spleen. Treatment with pellets containing corticosterone caused dose-related increases in the plasma concentrations of corticosterone, glucose, triglyceride, and cholesterol, an increase in liver size, and a decrease in the size of the bursa and spleen. Thus the effects of ACTH1-24 are probably almost entirely mediated by corticosterone. During the second week of treatment with ACTH or corticosterone the plasma corticosterone concentration was lower than during the first week. Replacing corticosterone implants at Day 7 did not cause plasma corticosterone concentration to return to that observed in the first week suggesting increased removal of the hormone from the circulation. This response resembles the stress-induced change in circulating corticosterone and may be part of the process of adaptation.