We have previously demonstrated that high levels of inducible nitric oxide synthase (iNOS) expression in hepatocytes required a combination of LPS and TNF-alpha, IL-1 beta, and IFN-gamma. The need for such a complex regulatory system seemed unwarranted based on the importance of NO in the liver. Therefore, we investigated whether individual cytokines could induce NO synthesis in hepatocytes and characterized some of the mechanisms involved. Rat hepatocytes were stimulated in vitro with escalating doses of TNF-alpha, IL-1 beta, or IFN-gamma. Only IL-1 beta induced high levels of iNOS mRNA and corresponding NO2- + NO3- production, and dexamethasone and cycloheximide blocked a majority of this response. Nuclear run-on experiments revealed that IL-1 beta upregulated iNOS gene transcription. IL-1 receptor antagonist protein (IL-1ra) competitively inhibited IL-1 beta-stimulated NO synthesis, implying activation through a cell-specific receptor. Rats injected with both LPS and IL-1ra showed decreased hepatic iNOS mRNA and plasma NO2- + NO3- compared with rats given LPS alone, indicating that IL-1 beta plays a role in regulating iNOS expression within the liver in vivo during endotoxemia. The soluble TNF receptor antagonist, PEG-(rsTNF-RI)2, also suppressed hepatic iNOS mRNA levels and plasma NO2- + NO3- increases, supporting a role for this cytokine in LPS-induced iNOS expression. Finally, IL-1 beta at high doses also induced iNOS mRNA and significant NO2- + NO3- production in cultures of primary human hepatocytes. These data indicate an important role for IL-1 beta in the regulation of hepatic NO synthesis.