The effects of indomethacin, a cyclooxygenase inhibitor, upon plasma concentrations of prostaglandin E2 (PGE2), the febrile response, and metabolic and hematological alterations induced by lipopolysaccharide (LPS) were studied. Experimental endotoxemia was provoked via i.p. injection of 1.0 mg E coli LPS/kg in rats (group A). Indomethacin was introduced/os (2.5 mg/kg) 30 min prior to LPS challenge (group B). Pretreatment with this medication completely inhibited the hyperthermic response to LPS and eliminated the LPS-induced non-specific symptoms of anorexia, adipsia, reduced locomotory activity and gastrointestinal troubles. Plasma PGE2 concentrations increased as early as the 2nd h after the LPS challenge but were blocked when endotoxin application was preceded by indomethacin treatment. Indomethacin did not significantly influence hematological parameters. The dynamics of hematocrit and erythrocyte counts were similar in both groups with a decrease up to the 2nd h followed by an increase to maximum at post-treatment day 3. Pretreatment with indomethacin did not influence the endotoxin-induced leukopenia observed at the 2nd h or the accompanying neutropenia and left shift. Cyclooxygenase inhibition affected total protein concentrations; they were decreased in the early hours of the study (hours 4-6) in both groups. The later tendency towards increase in total protein concentrations was more expressed in animals from group B. Changes in blood glucose were characterized by a permanent tendency towards decrease after hour 2 of LPS challenge up to day 6 (group A). In group B, a similar tendency was observed, but glucose concentrations decreased between hours 2-6 and then returned to initial values.