Hypothesis: Administration of a single dose of progesterone following trauma and hemorrhage in progesterone-deficient rats would ameliorate the inflammatory response and hepatocellular damage. Setting: A university laboratory. Interventions: Ovariectomized female Sprague-Dawley rats (250-350 g; Charles River Laboratories Wilmington, Mass) underwent a 5-cm midline laparotomy (ie, induction of soft tissue trauma), were bled to a mean arterial blood pressure of 35 mm Hg for about 90 minutes, and then were resuscitated using Ringer lactate solution. Progesterone (25 mg/kg of body weight) or vehicle was administered subcutaneously at the end of resuscitation. In additional animals, Kupffer cells were isolated following trauma, hemorrhage, and resuscitation and treated in vitro with progesterone, lipopolysaccharide, or both. Main Outcome Measures: Six hours following resuscitation, plasma tumor necrosis factor α (TNF-α) and interleukin 6 (IL-6) levels and liver myeloperoxidase activity were determined. Hepatocellular function (maximum velocity of indocyanine green clearance [Vmax] and the efficiency of the active transport or Michaelis-Menten constant [Km]) and plasma levels of transaminases were measured 20 hours after resuscitation. Kupffer cell IL-6 and TNF-α production were assessed. Results: Plasma levels of TNF-α, IL-6, aspartate aminotransferase, and alanine aminotransferase, as well as hepatic myeloperoxidase activity were increased, whereas indocyanine green clearance was depressed in vehicle-treated rats following trauma-hemorrhage. Animals treated with progesterone showed significantly reduced levels of the TNF-α, IL-6, and transaminases as well as reduced myeloperoxidase activity in the liver. Progesterone-treated animals showed increased Vmax and Kmax values for indocyanine green. In vitro treatment of Kupffer cells with progesterone decreased TNF-α production but did not affect the production of IL-6. Conclusion: Progesterone administration following trauma-hemorrhage ameliorates the proinflammatory response and, subsequently, the hepatocellular injury via direct action on immunocompetent cells.