Background: Perfusion of the skeletal muscle microvasculature in response to nutrition is thought to be central for muscle mass maintenance (via protein anabolism) and whole-body glucose homeostasis (glucose disposal), both of which can be compromised in a variety of diseases including diabetes, cancer cachexia, COPD and in ageing muscle. Objective: We set out to define whether enhancing muscle microvascular blood flow (MBF) could influence muscle protein turnover under postabsorptive and postprandial conditions in young men. Design: We recruited 10 young (23.2±2.1 y) men in whom we measured bulk [femoral arterial] leg blood flow (LBF), muscle microvascular blood flow (MBF) and muscle protein synthesis (MPS) under postabsorptive (12 h fasted) and postprandial conditions (I.V glamin (prime: 34 mg.kg-1, constant infusion: 102 mg.kg.hr-1); dextrose to sustain blood glucose between 7-7.5 mmol.l-1). In addition, we infused a pharmacological vasodilator (methacholine) into the artery of one leg to permit bilateral comparison of the effects of nutrition alone or nutrition coupled to pharmacological vasodilation. We measured LBF [femoral artery] by Doppler ultrasound and muscle MBF by contrast-enhanced ultrasound (CEUS) using DefinityTM perflutren contrast agent infused at a rate of 1.2 ml.min-1 (1.5 ml in 18.5 ml 0.9 % saline). After 9 min to allow attainment of steady state concentrations, DefinityTM microbubbles were destroyed by high mechanical index ultrasound and refilling of the microvascular space (MBF) was imaged during 4 consecutive 45 s recordings. Data was analysed using Q-lab quantification software. Rates of MPS and net leucine balance were measured using [1, 2-13C2] leucine tracer coupled to gas-chromatographic/ combustion mass-spectrometric techniques. Results: We observed increases in LBF (0.51±0.02 vs. 0.57±0.04 l.min-1, P<0.05), MBF (+25±10%, P<0.05), MPS (0.04±0.004 vs. 0.08±0.001 %.h-1, P<0.05) and positive leucine net balance in the postprandial condition (-9.72±4.76 vs. 40.31±12.27 nmol.min.100g lean leg mass-1, P<0.01). However, despite methacholine infusions augmenting nutrition-mediated increases in LBF (0.57±0.04 vs. 1.15±0.08 l.min-1, P<0.05) and MBV (+79±30%, P<0.05) this provided no enhancement of MPS (0.08±0.01 vs. 0.07±0.01 %.h-1) or leucine net balance (40.31±12.27 vs. 57.0±13.66 nmol.min.100g lean leg mass-1). Conclusion: We conclude that pharmacological enhancement of LBF and MBV above that of nutrition does not further augment muscle protein metabolism. This indicates that muscle nutritive blood flow is not a limiting factor for muscle protein metabolism, at least in young men.