Impaired vascular function in older age compromises the positive effects of nutrition on limb blood flow, perhaps impacting on maintenance of healthy muscle mass. Herein, we explored: (i) the effect of age upon macro and micro-vascular limb blood flow responses to nutrition, and (ii) the potential for arginine, a substrate of nitric oxide synthase, to improve nutrient-mediate vascular responsiveness in older people. Using contrast enhanced ultrasound (CEUS) with Sonovue™ SF6 microbubbles, along with phase shift Doppler, we measured: changes in leg blood flow (LBF), muscle microvascular blood volume (MBV), microvascular flow velocity (MFV) and their product microvascular blood flow (MBF), in response to a 15g oral mixed EAA feed in 8 young (~20y) and 8 older (~70y) men. An additional 8 older men (~70y) had the same 15g mixed EAA feed, supplemented with 3g L-arginine. Using 13C6-phenylalanine tracer, muscle protein synthesis (MPS) was also measured. Measurements were made 2hr before and 4hr after feeding. Young men showed marked vascular responses to EAA feeding with an early increase in MBV (45±13%, P<0.001) due to capillary recruitment. This was achieved by 45 min post feed, preceding changes in LBF, MFV and MBF, which were observed 120-180 min post feed. LBF increased 44±11% (P<0.001), MFV increased 78±31% (P<0.01) and MBF increased 130±52% (P<0.001) from fasting. In contrast, older men fed 15g EAA had no significant changes in LBF or any index of muscle microvascular flow. Older men receiving 15g EAA with 3g arginine displayed increases in LBF (25±5%, P<0.05), MFV (88±48%, P<0.001) and MBF (110±70%, P<0.05, all mean±SEM, 2-way RM ANOVA) though MBV remained unchanged from fasted. Fasting MPS rates were similar in young (0.054%/hr) and older men (0.052%/hr). Young men demonstrated a total synthetic response of 0.27% in 4hrs after 15g EAA feeding. Older men had a blunted anabolic response of 0.22% (P<0.05 vs. young) in the same period, which was not augmented by 3g arginine (0.20%, P<0.05 vs. young, 1-way ANOVA). Oral EAA feeding achieves early recruitment of muscle capillaries (increasing MBV) with little change in flow; this is consistent with dilatation of terminal arterioles and would promote nutrient delivery by increasing exchange surface area and decreasing capillary-to-fiber distance. The later increase in LBF, MFV and MBF is consistent with a proximal shift in the level of regulating vessels. That arginine restores late, but not early, microvascular responses to feeding suggest different underlying mechanisms. Arginine’s failure to augment MPS responses to feeding despite restoring LBF and MBF calls into question the physiological significance of the late circulatory response to feeding and promotes interest in early MBV changes.