To investigate the possible mediation by prostaglandins of changes in behaviour induced by peripheral injection of interleukin-1 (IL-1), two types of behavioural tests were chosen, social exploration in mice and schedule-controlled behaviour in rats. Mice treated with 1 and 2.5 micrograms recombinant human IL-1 beta showed a time- and dose-dependent decrease in exploration of a juvenile conspecific. This effect was completely blocked by pretreatment with 10 mg/kg indomethacin or 10 mg/kg piroxicam, but not with 50 mg/kg aspirin. The disruption of operant responding induced by 5 micrograms IL-1 in rats was also suppressed by pretreatment with 5 mg/kg indomethacin or 10 mg/kg piroxicam. These results indicate that prostaglandins mediate the behavioural effects of peripherally injected IL-1.