We have investigated the effects of serotonin depletion on immune-mediated activation of the hypothalamopituitary-adrenal (HPA) axis. Corticotrophin-releasing factor (CRF) mRNA, c-fos mRNA and Fos peptide responses in the paraventricular nucleus (PVN) together with circulating levels of corticosterone were assessed in response to i.p. injections of three doses of lipopolysaccharide (LPS) both in control animals and animals pretreated with p-chlorophenylalanine (PCPA).Conscious animals received either an i.p. injection of 0·5 ml saline or 200 mg/kg PCPA in 0·5 ml saline on 2 consecutive days. This treatment resulted in a 93% depletion of serotonin on the fourth day. On day 4, animals received i.p. injections of LPS (2·5 mg/0·5 ml saline, 250 µg/0·5 ml or 50 µg/0·5 ml; E. coli 055:B5), or saline injections as controls.Pretreatment with PCPA had no effect on the basal levels of corticosterone, or on the elevated levels induced by the three doses of LPS. Fos peptide and c-fos mRNA were undetectable in control animals, and Fos-like immunoreactivity increased in a dosedependent manner following i.p. LPS in both control and PCPA-pretreated animals. C-fos mRNA expression induced by LPS was unaffected by serotonin depletion. Following the lowest dose of LPS, CRF mRNA did not change above control levels, however, the medium and high doses of LPS produced a significant (P<0·05) increase in CRF mRNA levels in both depleted and intact animals.To confirm the temporal effects of serotonin depletion on activation of the HPA axis we collected plasma at 30 min, 1, 2, 3, 4, 5 and 6 h after LPS in both intact and serotonin-depleted animals. No significant differences in plasma corticosterone levels were found at any of the time points between intact and depleted animals.It appears that, at least under these experimental conditions, serotonergic inputs do not seem to play a major role in mediating the effects of LPS on changes in mRNA levels in the PVN or on the subsequent activation of the HPA axis.