The present study was carried out to compare the threshold doses of interleukin-1 beta (IL-1 beta) necessary to elicit febrile and adrenocorticotrophic hormone (ACTH) responses. The results show that intravenous injection of a small dose of IL-1 beta (0.2 micrograms/kg) did not stimulate ACTH secretion but induced a significant febrile response. However, intravenous injection of a higher dose of IL-1 beta (2.0 micrograms/kg) induced significant increases in plasma ACTH accompanying the fever. These results suggest that the rise in body temperature per se is not responsible for the ACTH response and that the threshold dose of IL-1 beta to induce the ACTH response is higher than that to elicit the febrile response.