The relation between smooth muscle membrane potential and contractile force was investigated in the rat tail artery to assess the importance of smooth muscle depolarization in the control of smooth muscle tone. Smooth muscle membrane potential and contractile force were measured simultaneously in isolated pieces of rat tail artery exposed to a range of concentrations of norepinephrine, 5-h1ydroxy-tryptamine, or raised external potassium. Potassium caused depolarization and contraction when the membrane was depolarized beyond -40 mV. Maximum contraction occurred at -19 mV, and farther depolarization gave no increase in contraction. Both norepinephrine and 5-hydroxytryptamine caused contraction and depolarization, but the relation between depolarization and contraction was not the same as when potassium was used. There was significant contraction when the membrane potential was more negative than - 50 mV, and the membrane potential was around -30 mV during maximal contractions. Although they acted on pharmacologically different membrane receptors, the relation between membrane potential and contraction was the same for norepinephrine and 5-hydroxytryptamine. Prazosin reduced the responses to norepinephrine but did not change the relation between membrane potential and contractile force. These results indicated that norepinephrine and 5-hydroxytryptamine binding to their respective receptors might activate the same sets of intracellular processes that subsequently caused both depolarization and contraction.