Administration of intekleukin-1 (IL-1) into the cerebral ventricles produces marked physiological and behavioral effects. However, the precise locations within the central nervous system that mediate these effects have not been determined. Previous studies indicated that IL-1 induces neurophysiological, neurochemical and neuroendocrine changes within the hypothalamic paraventricular nucleus (PVN). These findings suggest that the PVN is also involved in mediating the behavioral effects of IL-1. This hypothesis was tested by examining the effects of administration of IL-1Β (10 or 50 ng/rat) or saline, either intracerebroventricularly or into the PVN, on fever and several behavioral parameters. IL-1Β, administered into both locations, induced a comparable suppression of motor activity, reduction in food and saccharine consumption, and loss of body weight. The febrile response to IL-1Β, assessed by a biotelemetric system, was significantly greater following administration into the PVN than into the lateral ventricle. Additionally, IL-1 administration into adjacent thalamic locations had no febrile or behavioral effects. These findings suggest that the PVN may be one of the brain structures involved in mediating the response to IL-1.