Effects on turnover of vasopressin (AVP) in the hypothalamus and on secretion of pituitary hormones, catecholamines and insulin after intraperitoneal injection of recombinant interleukin-1 (beta) (IL-1) were investigated in male wistar rats. Intraper-itoneal administration of IL-1 in a dose (1 µg) that maximally activated pituitary-adrenal activity failed to alter plasma concentrations of prolactin, luteinizing hormone and melanocyte-stimulating hormone. Rats chronically cannulated in the right jugular veins showed a time-related increase in plasma corticosterone concentrations in response to intraperitoneal administration of IL-1 that lasted up to 4 h. In the same rats, plasma epinephrine (E) and norepinephrine (NE) concentrations were only slightly elevated (2-fold increase) at 30 min and at 1 h after IL-1 administration. Unlike in endotoxin-resistant C3H/HeJ mice, where IL-1 induces hypoglycemia, IL-1 did not affect plasma concentrations of glucose and insulin in Wistar rats. In the zona externa of the median eminence, IL-1 stimulated corticotropin-releasing factor (CRF) turnover at an approximate rate of 15%/h, but did not cause a concomitant change in AVP turnover as can be observed after insulin-induced hypoglycemia. Since half of the hypothalamic CRF neurons have been shown to costore AVP, the data favor the view of a selective effect of IL-1 on a subtype of CRF neurons. We conclude that pituitary-adrenal activation in response to II-1 is caused by CRF secretion from a subtype of CRF neurons (not storing AVP) in the rat hypothalamus. Furthermore, the small and transient increase of plasma E and NE may be caused by a presynaptic action of IL-1 on sympathetic nerves in immune and/or other organs or may involve central CRF projections regulating sympathetic outflow.