Background: This study evaluated the effect of 17β-estradiol (E2) administration on cardiovascular parameters in male rats after trauma-hemorrhage and for an extended period (3 hours) of severe hypotension, based on blood-pool single photon emission computed tomography imaging. Methods: After a 5-cm midline laparotomy, male Sprague-Dawley rats were injected intravenously with Tc-bovine serum albumin; the animals were then bled for >45 minutes to reach maximum bleedout (MBO; removal of 60% of the circulating blood volume). E2 (1 mg/kg body weight, 0.4 mL/kg) or vehicle (cyclodextrin [CD], 0.4 mL/kg) was injected intravenously at MBO; no additional fluid was administered for 3 hours. Imaging was performed continually for a maximum of 3 hour post-MBO. The percentages of injected dose in heart, brain, liver, and kidney were quantified from the imaging (n = 8/group) at 0 minutes, 30 minutes, 60 minutes, 90 minutes, 120 minutes, 150 minutes, and 180 minutes post-MBO. Mixed model analysis of variance was used to analyze the difference between the two groups over six imaging time points (30-180 minutes post-MBO). Results: The percentages of injected dose of Tc-bovine serum albumin in heart, kidney, and liver after E2 administration were significantly higher than those after CD administration (p = 0.036, 0.025, and 0.028 for heart, kidney, and liver, respectively), whereas those in brain were not different between E2 and CD administration (p = 0.343). Conclusions: The significantly larger blood volume maintained in heart, kidney, and liver of rats after E2 therapy compared with control supports the notion that E2 produces salutary effects on the cardiovascular system after trauma-hemorrhage and even extended periods of severe hypotension. Copyright © 2010 by Lippincott Williams & Wilkins.