In this study the effect of immune system stimulation and intracerebroventricular (i.c.v.) administration of interleukin-1 beta (IL-1 beta) on hippocampal serotonergic neurotransmission, behavioral activity, and the hypothalamic-pituitary-adrenocortical (HPA) axis is described. An in vivo microdialysis method was used to measure hippocampal extracellular concentrations of serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in conscious, freely moving rats. In addition, we established a method to continuously monitor free corticosterone levels in dialysates. Behavioral activity was scored by measuring the time during which rats were active (locomotion, grooming, eating, drinking). We found a significant, positive relationship between behavioral activity and hippocampal extracellular concentrations of 5-HT. Intraperitoneal (i.p.) administration of the bacterial endotoxin lipopolysaccharide (LPS; 30, 100, and 300 micrograms/kg body weight) produced an increase in the extracellular concentrations of 5-HT and 5-HIAA in the hippocampus, which was paralleled by a significant decline in behavioral activity and a marked increase in extracellular corticosterone levels. Thus, the close correlation between hippocampal extracellular 5-HT levels and behavioral activity observed in control rats was disrupted in the LPS- treated animals. The effects of i.p. LPS could be mimicked by i.c.v. application of recombinant human IL-1 beta (hIL-1 beta; 100 ng). i.c.v. pretreatment with the IL-1 receptor antagonist (IL-1ra; 10 micrograms) antagonized the hIL-1 beta-induced effects. IL-1ra showed no intrinsic effects. Furthermore, it was found that i.c.v. pretreatment with IL-1ra (10 micrograms) significantly attenuated the i.p. LPS-induced (100 micrograms/kg body weight) rise in hippocampal extracellular 5-HT levels.