We examined the relationships between erythrocyte (RBC) composition (Na+,K+, and water content) and ouabain-insensitive transports (Na+-K+cotransport, Li+-Na+countertransport) and forearm vascular bemodynamks under standardized basal conditions and during vasoconstriction (intra-arterial infusion of graded doses of norepinephrine and angiotensin II) and vasodilation (intraarterial phentolamme and postischemic exercise). RBC water content correlated positively and significantly (r= 0.53,p= 0.001) with minimum forearm vascular resistance, a measure of vascular structural change, and negatively with nundinal forearm blood flow (r= − 0.55,p< 0.001). Similar correlations with forearm vascular resistance and blood flow were observed under all experimental conditions. RBC Na+-K+cotransport correlated positively and significantly (r= 0.43,p= 0.01) with the change in forearm blood flow produced by phentotamine, a functional measure of α-adrenergk tone, and was as strong an independent predictor of phentolamine-induced blood flow change as was arterial norepinephrine concentration. RBC Na+-K+cotransport was also significantly positively correlated with residual forearm blood flow and resistance after phentolamine administration, where nonadrenergk influences predominate. RBC water correlated negatively with LI+-Na+countertransport (r= −0.33,p< 0.05) and Na+-K+cotransport (r= −0.44,p< 0.01). We propose that RBC water is a marker for a vascular structural property that contributes to vascular reactivity. RBC Na+-K+cotransport seems to relate most strongly to the sympathetically mediated control of forearm blood flow and may also be linked to the intrinsic myogenk tone of the forearm vasculature. Li+-Na+countertransport is not significantly correlated with vascular hemodynamk measures but may, along with Na+-K+cotransport, relate to the control of cell composition.