To date, the neurotransmitter(s)and pathways involved in cutaneous active vasodilatation are not fully understood. The purpose of this study was to determine the contribution of neurokinin-1 (NK₁) receptors and, indirectly, substance P, to active vasodilatation. Our experimental model exploited our previous findings that repeated microdialysis infusions of substance P desensitise the NK1 receptors and that substance P-induced vasodilatation contains a substantial NO component. Eleven subjects were equipped with four microdialysis fibres on the ventral forearm. Site 1 served as a control and received a continuous infusion of Ringer’s solution. Site 2 received a continuous infusion of 10mM L-NAME to inhibit nitric oxide (NO) synthase. Site 3 received a 10µM dose of substance P to desensitise the NK₁ receptors prior to whole body heating. Site 4 received a 10µM dose of substance P combined with 10mM L-NAME to investigate possible interactions between NK₁ receptors and NO in active vasodilatation. Red blood cell (RBC) flux was measured via laser-Doppler flowmetry and cutaneous vascular conductance (CVC) was calculated as RBC flux/mean arterial pressure and normalized to maximal vasodilatation via 28mM sodium nitroprusside. Substance P was infused for 15 minutes at 4µl min^-1 in sites 3 and 4 and skin blood flow was allowed to return to baseline (~45-60 minutes). Subjects then underwent a period of whole body heat stress to raise oral temperature 0.8-1.0°C above baseline. Pre-treatment with substance P increased CVC to 48±2 %CVCmax, which was significantly greater than sites pre-treated with substance P combined with L-NAME (27±2%CVCmax; P<0.001). During whole body heating, CVC in control sites increased to 69±3 %CVCmax. Sites pre-treated with substance P (48±3 %CVCmax) were significantly reduced compared to control sites (P<0.001). The CVC response to whole body heat stress in L-NAME sites was significantly reduced (32±3 %CVCmax; P<0.001) compared to both control sites and sites pre-treated with substance P. The CVC response to whole body heating was nearly abolished in sites pre-treated with substance P combined with L-NAME (20±2 %CVCmax) and was significantly reduced compared to the other three sites (all P<0.001). These data suggest NK₁ receptors contribute to active vasodilatation and that combined NK₁ receptor desensitisation and NO synthase inhibition further diminshes active vasodilatation.