Isolated non-stripped segments of the rat tail artery, after stimulation by catecholamines or high-K, developed an isometric contraction strong enough to as, to allow easy recording of complete dose response curves. Lowering external Na to 17% of its standard concentration decreased the responses to the higher range of K concentrations whereas it enhanced those to the lower range of K concentrations. Raising external Na by 38% or decreasing it 50% did not alter responses to adrenaline. But, lowering Na enhanced those responses when arteries were rendered hypodynamic by prior immersion in a low-Ca medium. The curves depicting the relationship between Ca concentrations and tension induced by adrenaline or high-K were also studied. Curves of the arteries exposed to adrenaline were placed to the left with respect to those of K. The present data provide evidences that Na is involved in the excitation-contraction coupling processes of arterial smooth muscle. However, its role is affected when Na concentration is far below the physiological one. The effects of changing Na concentrations vary according to the K and Ca concentrations. It also reinforced the belief that adrenaline may act by increasing the Ca inward movement beyond that elicited by mere depolarization.