Objectives: To investigate the effects of S-ethylisothiourea (S-EITU) on hemodynamics, oxygen transport, and regional blood flow in healthy and septic sheep. Design: Prospective, randomized, controlled experimental study with repeated measures. Setting: Investigational intensive care unit at a university medical center. Subjects: Eleven healthy, female adult sheep of the Merino breed, divided into a control group (n = 5) and into a group treated with S-EITU (n = 6). Interventions: All sheep were chronically instrumented. After a 5-day recovery period, they were randomly assigned to either control or S-EITU groups. While control sheep received only saline, S-EITU was administered in increasing doses at 1, 3, and 9 mg/kg/hr over 1 hr each (nonseptic phase). After 2 days of recovery, a continuous infusion of live Pseudomonas aeruginosa (2.5 x 10(6) colony-forming units/min) was started in all sheep and maintained for the remainder of the experiment. After 24 hrs of sepsis, the sheep again received their assigned treatment (septic phase). in both the nonseptic and septic phases, the sheep received colored microspheres through a left atrial catheter to allow analysis of regional blood flows. All animals were autopsied at the end of the experiments, and organ probes were removed for blood flow analyses. Measurements and Main Results: The administration of S-EITU caused a dose-dependent vasoconstriction in the nonseptic phase. After 24 hrs of Pseudomonas infusion, all sheep developed a hyperdynamic circulatory state, with Increased cardiac indices and reduced arterial pressures and systemic vascular resistances. Oxygen extraction decreased significantly, preventing an increase in oxygen consumption, despite an increased oxygen delivery. The hyperdynamic circulation was dose dependently reversed by S-EITU, causing an increase In arterial pressure by peripheral vasoconstriction. Sheep in the central group showed a continuation of the hyperdynamic circulation. The effects of S-EITU on hemodynamics and regional blood flows were comparable under septic and nonseptic conditions. Conclusions: With the inducible form of nitric oxide synthase expressed under septic, but not under nonseptic conditions, S-EITU was expected to have vasoconstrictive properties only in the septic phase. It produced a comparable vasoconstriction during the nonseptic phase of the experiment. Thus, either S-EITU does not selectively block the Inducible nitric oxide synthase in sheep, or other vasodilators besides nitric oxide play an important role in septic vasodilation.