NEW FINDINGS: • What is the central question of this study? Quantitative values of shear rate-specific blood viscosity and shear stress in the human macrovasculature in response to exercise hyperaemia are unknown. • What is the main finding and its importance? Using the handgrip exercise model, we show that an increase in brachial artery shear rate led to a decrease in blood viscosity, despite concomitant haemoconcentration. This shear-thinning behaviour of blood, secondary to increased erythrocyte deformability, blunted the expected increase in brachial artery shear stress based on shear rate prediction. Our data yield new insights into the magnitude and regulation of macrovascular blood viscosity and shear stress under physiological conditions of elevated metabolic demand and blood flow in humans. ABSTRACT: Blood viscosity is a well-known determinant of shear stress and vascular resistance; however, accurate quantitative assessments of shear rate-specific blood viscosity in the macrovasculature under conditions of elevated blood flow are inherently difficult, owing to the shear-thinning behaviour of blood. Herein, twelve men performed graded rhythmic handgrip exercise at 20%, 40%, 60% and 80% of their maximal workload. Brachial artery shear rate and diameter were measured via high resolution Duplex ultrasound. Blood was serially sampled from an intravenous cannula in the exercising arm for the assessment of blood viscosity (cone-plates viscometer). We measured ex vivo blood viscosity at ten discrete shear rates within the physiological range documented for the brachial artery under basal and exercise conditions. Subsequently, the blood viscosity data was "fitted" with a two-phase exponential decay, facilitating interpolation of blood viscosity values corresponding to the ultrasound-derived shear rate. Brachial artery shear rate and shear stress increased in a stepwise manner with increasing exercise-intensity, reaching peak values of 940 ± 245 s-1 and 3.68 ± 0.92 Pa, respectively. Conversely, brachial artery shear rate-specific blood viscosity decreased with respect to baseline across all exercise-intensities by ∼6-11%, reaching a minimum value of 3.92 ± 0.35 mPa∙s, despite concomitant haemoconcentration. This shear-thinning behaviour of blood, secondary to increased erythrocyte deformability, blunted the expected increase in shear stress based on shear rate prediction. Consequently, the use of shear stress yielded a higher slope for the brachial artery stimulus versus dilation relationship than shear rate. Collectively, our data refute the use of shear rate to infer arterial shear stress-mediated processes. This article is protected by copyright. All rights reserved.