In an attempt to help elucidate the mechanisms involved in the contractile processes of vascular smooth muscle, the transport kinetics of Na⁺ and K⁺ in arterial walls （branches of canine femoral artery） were studied with isolated sections of vessels mounted on stainless steel rods. The experimental tracer washout data were analyzed based on a mathematical model, which takes into account diffusion in the extracellular space and transport across the cell membranes by both passive and active mechanisms, yielding values for the membrane properties. Values were obtained for the diffusion coefficient of the ions in the extracellular space (8.33×10^-6 cm²/sec at 37 ℃ and 6.88×10^-6 at 21.2 ℃ for Na⁺ and 1.23×10^-5 at 37 ℃ for K⁺), the membrane permeabilities to the ions (0.60×10^-8 cm/sec at 37 ℃ and 0.52×10^-8 at 21.2 ℃ for Na⁺ and 3.3×10^-8 at 37 ℃ and K⁺), the rate of active transport of ions through the cell membranes (2.09×10^-9 mEq/cm²sec at 37 ℃ and 1.52×10^-9 at 21.2 ℃ for Na⁺ and 0.82×10^-9 at 37 ℃ for K⁺), and the intracellular ion concentrations (18.7 mEq/l at 37 ℃ and 27.5 at 21.2 ℃ for Na⁺ and 126 at 37 ℃ for K⁺). The results indicate that the Na⁺ - K⁺ pump is electrogenic with a coupling ratio in the order of 2 to 1. This is consistent with the hypothesis that an electrogenic Na⁺ - K⁺ pump plays a role in maintaining the cell potential and in regulation of vascular resistance to blood flow.