Ouabain and K-free solution affected the tension development of isolated vessels as follows. In rabbit aorta, ouabain induced a transient contraction followed by a delayed sustained contraction whereas K-free solution was almost ineffective; both ouabain and K-free solution produced a sustained contraction in guinea pig aorta; rat aorta rarely responded to ouabain but developed a contraction in K-free solution. Phentolamine inhibited the transient contraction produced by ouabain in rabbit aorta. In rabbit aorta, ouabain produced a concentration-dependent potentiation of norepinephrine contraction. In denervated rabbit aorta, however, the potentiation only occurred in the presence of 5 X 10(-7) to 5 X 10(-6) M ouabain. K-free solution inhibited the norepinephrine contraction in denervated rabbit aorta. Ouabain potentiated the norepinephrine contraction in guinea pig aorta but not in rat aorta. It is concluded that ouabain and K-free solution affect contraction both indirectly via the release of endogenous catecholamines and directly by acting on vascular smooth muscle, and that there are species differences in the sensitivity to ouabain.