Estrogen increases nitric oxide (NO) production by inducing the activity of endothelial NO synthase (eNOS) (Simoncini et al. Nature 407: 538, 2000). Ischemia (30 min) and reperfusion (I/R) increased the number of adherent leukocytes and decreased their rolling velocities in mouse cremaster muscle venules with a strong dependence on wall shear rate. Minimum rolling velocity at ∼5 min after the onset of reperfusion was accompanied by increased P-selectin expression. This preceded the peak in leukocyte adhesion (at 10–15 min). In untreated wild-type mice, I/R caused a decrease of leukocyte rolling velocity from 37 to 26 μm/s and a 2.0-fold increase in leukocyte adhesion. Both were completely abolished by 0.25 mg ip estrogen 1 h before surgery. In eNOS^−/− mice, the decrease of leukocyte rolling velocity and increase in adhesion were similar but were only marginally improved by estrogen. We conclude that the protective effect of estrogen, as measured by leukocyte rolling and adhesion, is significantly reduced in eNOS^−/− mice, suggesting that induction of eNOS activity is the major mechanism of vasoprotection by estrogen in this model.