The role of calcium present in adventitial and media-intimal layers of rabbit aorta in the fast (F-) and the slow (S-) components of norepinephrine (10-5m) contraction was evaluated in this study. An increased sensitivity and decreased maximum tension to response to norepinephrine was observed in adventitia-free aorta as compared to whole aorta. Incubation in calcium-free Ringer solution with or without EDTA decreased the F-component of norepinephrine contraction of the adventitia-free aorta, whereas the F-component of the whole aorta was not affected. The S-component of both preparations was abolished in calcium-free Ringer solution. Norepinephrine increased calcium uptake into whole and adventitia-free aortic strips. An increase in calcium uptake induced by norepinephrine during the F-component of norepinephrine contraction was also observed. 45Ca efflux studies revealed the presence of an exchangeable calcium fraction in the adventitial layer. These data suggest that the F-component of norepinephrine contraction is partially dependent on the extracellular calcium distributed in the adventitial layer of rabbit aorta.