The purpose of this study was to investigate the role of Rac1 and estrogen in sex difference of cardiac tumor necrosis factor-alpha (TNF-alpha) expression during endotoxemia. Endotoxemia was induced in male and female mice by peritoneal injection of lipopolysaccharide (LPS, 4 mg/kg). Compared with female mice, male mice produced more TNF-alpha in the heart 4 h after LPS treatment, which were correlated with higher Rac1 and NADPH oxidase activity, more phosphorylation of ERK1/2 and p38 MAPK, and up-regulation of toll-like receptor-4 (TLR-4) expression in male mice. Cardiac specific Rac1 knockout or administration of 17beta-estradiol down-regulated Rac1 expression, attenuated gp91(phox)-NADPH oxidase expression and activity, decreased phosphorylation of ERK1/2/p38 MAPK and inhibited cardiac TNF-alpha expression induced by LPS, suggesting an important role of Rac1 and estrogen in LPS-stimulated TNF-alpha expression in the heart. More importantly, the sex difference in TNF-alpha expression was abrogated by Rac1 knockout or gp91(phox) knockout and by administration of apocynin or N-acetylcysteine in LPS-stimulated mice. To investigate the functional significance of sex difference in endotoxemia, heart function was measured in isolated hearts with a Langendorff system. Male mice exhibited worse myocardial dysfunction compared with female in endotoxemia. Treatment of male mice with 17beta-estradiol attenuated myocardial dysfunction during endotoxemia. In conclusion, LPS induces Rac1 activation, which contributes to NADPH oxidase activity and phosphorylation of ERK1/2/p38 MAPK, leading to TNF-alpha expression in the heart. The sex difference in TNF-alpha expression is estrogen-dependent and mediated via Rac1 dependent NADPH oxidase/ERK1/2 and p38 MAPK pathway in LPS-stimulated hearts.