While interleukin-1 (IL-1) is intimately involved in locally modulating the acute inflammatory response, it is also able to influence processes at remote sites, i.e., in an endocrine manner. While there is as yet little evidence that IL-1 can cross the blood-brain barrier, many effects such as fever, increased slow-wave sleep, anorexia and the modulation of neuroendocrine function suggest an action of circulating IL-1 at regulatory sites within the hypothalamus. However, there is accumulating evidence for IL-1 originating within the central nervous system (CNS), and it is currently unclear as to whether the neurally mediated manifestations of the acute inflammatory response are due to activation of central or peripheral (circulating) IL-1. In this study we have characterized the release of IL-1 from rat hypothalamic explants, and we have investigated the effects of putative modulators of IL-1 release, lipopolysaccharide (LPS) and the prostaglandins E<sub>2</sub> (PGE<sub>2</sub>) and F<sub>2α</sub> (PGF<sub>2α</sub>). After 1 h of incubation, IL-1-like activity in hypothalamic supernatants ranged between 175 and 2,304 munits/mg of protein; this was substantially inhibited by the addition to the bioassay system of antibodies (1:200) against IL-1α, but not against IL-1β. LPS and PGE<sub>2</sub> significantly stimulated IL-1 release at 100 and 1 ng/ml respectively, whereas PGF<sub>2α</sub> had no effect in the range of doses tested. It is therefore concluded that the control of hypothalamic IL-1 release may be investigated by means of acute rat hypothalamic explants. Furthermore, both LPS and PGE<sub>2</sub> can stimulate IL-1 release in this model, although no clear dose-response relationship could be discerned. Taking into account previous studies which have shown that IL-1 may specifically stimulate the release of hypothalamic PGE<sub>2</sub>, these data suggest that (1) PGE<sub>2</sub> may mediate communication between circulating and CNS-derived IL-1, and (2) the effects of LPS on neuroendocrine function may occur, at least in part, independent of circulating IL-1.