Cumulative dose-response curves to angiotensin II were performed on helical strips from canine lateral saphenous vein. Threshold concentrations were in the range of 10(-18)-10(-17)M. Increases in angiotensin from 10(-17)-10(-12)M failed to elicit further increases in tension. Subsequent increases in angiotensin concentration from 10(-11)-10(-7)M again produced dose-related increases in tension. Repeated dose-response curves in the same strips showed reduced maximal response. Responses to low concentrations of angiotensin were attenuated by low concentrations of phentolamine. These results suggest that, at extremely low concentrations angiotensin produced marked contractions in canine saphenous vein strips by releasing endogenous norepinephrine from sympathetic nerve terminals.