Peripheral and central injections of recombinant rat interleukin-1 beta (IL-1 beta) have been shown to decrease social exploration in rats. To test the involvement of vagal afferents in the communication between the immune system and the brain, sham-operated and vagotomized rats were injected peripherally or centrally with physiological saline or IL-1 beta 4 weeks after surgery. Vagotomy attenuated the depression in social exploration induced by i.p. administration of IL-1 beta (15 micrograms) but did not alter the behaviour-depressing effects of an intracerebroventricular (i.c.v.) injection of IL-1 beta (45 ng). These results confirm the role of vagal afferent nerves in the transmission of an immune message from the periphery to the brain, and show that vagotomy does not impair the direct sensitivity of the brain itself to immune signals.