The thiol-containing compound dimethylthiourea (DMTU) is a known protectant in various models of oxidant-mediated tissue damage. Protective effects of DMTU have also been reported in studies on endotoxin-induced (LPS-induced) tissue injury. DMTU may exert this protective effect by reducing oxidative stress. In this study we investigated the effect of DMTU on survival, oxidative stress, and tumor necrosis factor (TNF) activity in two rat models of gram-negative bacterial sepsis. Intraperitoneal injection of 500 mg DMTU/kg protected against the lethal effects of intraperitoneally injected LPS (5 mg/kg) and live Salmonella typhimurium (3.3 x 10(10) CFU/kg). LPS injection resulted in oxidative stress, as indicated by an elevated concentration of hydrogen peroxide (H(2)O(2)) in normal and carbon monoxide-treated deproteinized blood. We also observed increased H(2)O(2) levels in animals injected with live Salmonella typhimurium. Although DMTU improved survival in both models, H(2)O(2) concentrations were not affected by it. This is consistent with our in vitro observation that DMTU is a weak H(2)O(2) scavenger. Serum TNF activity, however, was substantially decreased by DMTU, and this was associated with a reduced activation of nuclear factor kappaB in the peritoneal cells of LPS-treated rats. In addition, LPS-induced TNF production in vitro by rat peritoneal macrophages was inhibited by DMTU (p < 0.05). These results suggest that the protective effect of DMTU in gram-negative bacterial sepsis may be the result of a reduction in TNF activity. DMTU does not exert this effect by H(2)O(2) scavenging but may inactivate toxic H(2)O(2) metabolites.