Introduction: Sex hormones underlie the lower incidence of cardiovascular disease in pre-menopausal women. Vascular inflammation is involved in the pathogenesis of several cardiovascular diseases and is dependent, in part, on immune cell recruitment, for which leukocyte rolling is a crucial step. We have demonstrated that a sex-specific prevalence of endothelium-derived hyperpolarising factor (EDHF) underlies the protection of females against hypertension (1). Herein, we assessed whether sex differences in leukocyte rolling might contribute to cardioprotection in females and the potential role of EDHF. Material and Methods: Wild-type (WT;C57BL/6), endothelial NO synthase knockout (eNOS KO), cyclooxygenase-1 (COX-1) KO and eNOS/COX-1 double KO (dKO) mice, and ovariectomized (OVX) and sham-operated control (SHAM) animals (15-20g) were anaesthetised. The mesenteric vascular bed was exteriorised and prepared for intra-vital microscopy. Mice were pretreated with saline or interleukin-1β (IL-1β,5ng,i.p.) 90min or 4h prior to analysis. One to three randomly selected post-capillary venules (diameter 20-40μm,length>100μm) were observed for each mouse; measurements were taken 5–10min after exposure of the chosen vessels. The extent of basal and IL-1β-elicited leukocyte rolling was analyzed by counting the number of cells passing a fixed point per minute. Results: Basal leukocyte rolling in male eNOS KO, COX-1 KO and dKO was elevated compared to female WT (30.3±5.5,30.7±8.8 and 25±5.5 vs 12.6±2.8, respectively;n≥8;all P<0.05). The same profile was apparent in female COX-1 KO mice (24.3±2.9 vs 4.7±0.7;n≥6;P0.05). In IL-1β-treated animals (90min), leukocyte rolling was significantly increased in male COX-1 KO mice compared to WT mice (72.6±16.2 vs 31.2±9.7;n≥4;P<0.05) while in female eNOS KO, COX-1 KO and dKO mice the rolling was significantly reduced compared to males with the corresponding genotype (18.2±3.6 vs 39.7±11.3,20.1±8.1 vs 72.6±16.2 and 13.9±6.1 vs 35.5±12.5,respectively;n≥4;P<0.05). In IL-1β-treated dKO, this difference persisted at 4h (male 13.2±4.1 vs female 5.7±0.7;n≥4;P<0.05). OVX mice showed an increase in leukocyte rolling compared to control SHAM (26.1±10 vs 10.4±1.8;n=5;P<0.05). Conclusion: These results indicate that females show a reduction in basal and IL-1β-stimulated leukocyte rolling compared to males. Basally, leukocyte rolling in females is modulated by COX-1; however, in the presence of an inflammatory stimulus this control is exerted by EDHF. Thus, sex hormones might be implicated in female protection against vascular inflammation.