1. Elevation of [K(+)](e) induced a contraction of rabbit aorta. If 10 mM-La(3+) was applied to rabbit aortae prior to [K(+)](e) elevation no contraction occurred. When 10 mM-La(3+) was applied simultaneously with, or at short time periods after, elevation of [K(+)](e) graded contractions were obtained whose magnitudes were higher if La(3+) was added at a later stage.2. Net cellular Ca fluxes associated with these graded contractions were measured using (45)Ca as well as atomic absorption. Correlation of the graded contractions with the net inward Ca movements yielded a Ca activation curve for intact arterial smooth muscle cells. The curve was S-shaped with half-maximal activation at a net Ca uptake of 41 mumole/kg wet wt. of aorta and full activation at 64 mumole/kg. If corrections were made for the extracellular space, full activation occurred at a net Ca uptake of 102 mumole/kg smooth muscle cells or roughly 25 mumole in excess of the value calculated to bind to the activation sites on arterial smooth muscle myosin.3. The contractile force of the smooth muscle cells not only depended on the magnitude, but also on the rate of the net Ca uptake.4. Elevation of tissue c-AMP through inhibition of phosphodiesterase greatly reduced both the rate and magnitude of contraction without affecting the size of the high K(+) stimulated Ca influx. It was concluded that high K(+)-induced depolarization activates the rabbit aorta by stimulating a net Ca uptake, and that the degree of activation is modified by c-AMP sensitive Ca accumulation by intracellular organelles.