To verify the effect of immunosuppressants on the endotoxin-induced increase in iNOS activity, the action of immunosuppressants, dexamethasone (1.5 mg/kg), azathioprine (5 mg/kg/day) and cyclosporine (10 mg/kg), were evaluated in mice pretreated with LPS. The intraperitoneal injection of lipopolysaccharide (10 mg/kg) increased the nitric oxide synthase (NOS) activity in the brain and liver to maximum at 1 and 3 hours, respectively. The increase in NOS activity was blocked by the treatment with NOS inhibitor, LNAME(300 mg/kg) and aminoguanidine(100 mg/kg); a protein inhibitor, cycloheximide (10 mg/kg); and a transcription inhibitor of inducible NOS(iNOS), dexamethasone(1.5 mg/kg). Immunosuppressants, azathioprine (5 mg/kg) and cyclosporine (10 mg/kg), effectively blocked the increase in NOS activity. These results suggest that iNOS expression plays an important role in LPS-induced the increase in NOS activity and that immunosuppressants can be used as candidate for therapeutic agents in endotoxemia.