The effects of hormonal agonists (norepinephrine, angiotensin, and histamine) on 45Ca efflux from the rabbit aorta were studied using a Ca-EGTA buffered efflux medium. Each caused a transient stimulation of efflux rate which probably reflected the release of an intracellular 45Ca store. The size of the stimulation of efflux correlated with the size of the initial rapid phase of contraction. The norepinephrine-sensitive intracellular Ca fraction was estimated to be greater than 21 mumoles/Kg wet tissue weight. This fraction is separate from intracellular Ca which is accumulated during relaxation. Evidence is presented for the lack of cyclic nucleotide involvement in the release of Ca2+, and possible alternative modes of coupling are discussed.