Maintained reduction in extracellular K+ concentrations [( K]e) of isolated canine mesenteric arteries produced two contractions separated by a period of several hours. The mechanisms for respective early and late contractions were investigated. First, early responses to reduced [K]e were examined quantitatively over a period of 2 hr. Critical [K]e for initiating the contraction was 0.14 mM. Early contractions induced by K-free solution were abolished by phentolamine (10(-6) M) or reserpine pretreatment (0.5 mg/kg) but were not inhibited by cocaine (3 X 10(-6) M) or bretylium (2 X 10(-5) M). Treatment with ouabain at a low concentration of 2 X 10(-7) M potentiated markedly the phentolamine-sensitive contractile response to reduced [K]e. In contrast, contractile responses to norepinephrine, tyramine and transmural electrical stimulation were not potentiated by ouabain. Second, the late contraction, that occurred 4 to 5 hr after K-free substitution, also occurred in arteries pretreated with phentolamine or reserpine. The phentolamine-resistant late contraction was not inhibited by atropine, chlorpheniramine, cimetidine, saralasin, aspirin or methysergide. Ouabain potentiated the late contractile response to reduced [K]e in the presence of phentolamine. Thus, in canine mesenteric arteries, reduction in [K]e causes an early contraction due to norepinephrine release from adrenergic nerves and a late contraction that appears to be of myogenic origin. Ouabain may enhance both neurogenic and myogenic derived contractions in response to reduced [K]e by exaggerating the inhibition of the Na+,K+ exchange pump in the membranes of adrenergic nerves and vascular smooth muscle.