The effects of neomycin (7.0 mM) on 45Ca movements and distribution were investigated in canine aortae and in canine carotid and terminal mesenteric arteries. Uptake of <sup>45</sup>Ca was measured in calcium-free solution; the <sup>45</sup>Ca tissue spaces in the carotid and terminal mesenteric arteries were 2–4 times greater than those observed in the aorta. Exposure of the aortae and the terminal mesenteric arteries to 1.5 mM Ca<sup>++</sup> during the washout elicited large increases in <sup>45</sup>Ca efflux in both preparations (increase in terminal mesenteric > aorta). Moreover, in all three arterial preparations, neomycin reduced <sup>45</sup>Ca uptake and induced a sustained increase in <sup>45</sup>Ca efflux (effects on terminal mesenteric > carotid > aorta). The terminal mesenteric and carotid arteries may accumulate and bind <sup>45</sup>Ca at superficial membrane sites (readily exchangeable <sup>45</sup>Ca) to a greater degree than does the aorta. If Ca<sup>++</sup> located at these membrane sites contributes directly to the maintenance of mechanical responsiveness, then agents which alter membrane binding of Ca<sup>++</sup> (e.g. neomycin) may exert a stronger action on these highly reactive vessels. Thus, contractile responsiveness in peripheral arteries may depend upon depots of superficially bound Ca<sup>++</sup> to a greater degree than in the more centrally located aorta.