The successful therapeutic use of interferon-alpha (IFN-alpha) in myeloproliferative disorders offered the possibility to test its acute and long-term effects on the hypothalamic-pituitary-adrenal (HPA) axis in humans. ACTH and cortisol plasma concentrations were measured in 8 patients hourly starting from 4 p.m. through 12 p.m. on three occasions. The first time all patients were studied before initiation of therapy, when the vehicle was injected alone. The patients were studied again on day 1 of IFN-alpha therapy (5 million units) and once more after 3 weeks of therapy. On the control day, plasma concentrations of ACTH and cortisol were in the range expected for this time of day. In contrast, after the first administration of IFN-alpha a significant stimulation of the HPA axis was observed. After 3 weeks of IFN-alpha therapy, no significant stimulation of the HPA axis occurred after administration of IFN-alpha. IFN-alpha-induced adaptive changes in the HPA axis were also indicated by a significantly enhanced ACTH and cortisol response to exogenously administered supramaximal doses of corticotropin-releasing hormone (CRH) when the patients had been on IFN-alpha treatment for 3 weeks. To determine the exact locus of the IFN-alpha action, in vitro experiments were performed using rat hypothalamic organ and primary pituitary and adrenal cell culture systems. Thereby a significant stimulation of hypothalamic CRH secretion and rat adrenal corticosterone production was observed after INF-alpha at concentrations of 5 x 10(-8) M or 10(-7) M respectively. In contrast, no direct IFN-alpha effect on pituitary ACTH secretion could be observed in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)