Abstract Transient reduction in vascular function following systemic large muscle group exercise has previously been reported in humans. The mechanisms responsible are currently unknown. We hypothesised that sympathetic nervous system activation, induced by cycle ergometer exercise, would contribute to post‐exercise reductions in flow‐mediated dilation (FMD). Ten healthy male subjects (28 ± 5years) undertook two 30 minute sessions of cycle exercise at 75% HRmax. Prior to exercise, individuals ingested either a placebo or an α1‐adrenoreceptor blocker (Prazosin; 0.05mg.kg−1). Central hemodynamics, brachial artery shear rate (SR) and blood flow profiles were assessed throughout each exercise bout and in response to brachial artery FMD, measured prior to‐, immediately after, and 60‐minutes post‐exercise. Cycle exercise increased both mean and antegrade SR (P < 0.001) with retrograde SR also elevated under both conditions (P < 0.001). Pre‐exercise FMD was similar on both occasions, and significantly reduced (27%) immediately following exercise in the placebo condition (t‐test, P = 0.03). In contrast, FMD increased (37%) immediately following exercise in the Prazosin condition (t‐test, P = 0.004, interaction effect P = 0.01). Post‐exercise FMD remained different between conditions after correction for baseline diameters preceding cuff deflation and also post‐deflation shear rate. No differences in FMD or other variables were evident 60‐minutes following recovery. Our results indicate that sympathetic vasoconstriction competes with endothelium‐dependent dilator activity to determine post‐exercise arterial function. These findings have implications for understanding the chronic impacts of interventions, such as exercise training, which affect both sympathetic activity and arterial shear stress. This article is protected by copyright. All rights reserved