Scatchard plots of Ca2+ incorporation in canine aortic media-intima microsomes indicate 2 types of binding sites. Low affinity binding sites equilibrate more rapidly. Ca2+ incorporation is higher in a NaCl medium than in a KCl medium at a Ca2+ concentration in which high affinity binding predominates. The high affinity 45Ca binding is largely ATP-dependent. Efflux of non-superficial 45Ca from the microsomes is linear during a 32-min washout into a solution containing no added Ca2+ and is increased by addition of 0.1 mM EGTA [ethylene glycol bis(.beta.-aminoethyl ether) N,N,N',N'-tetraacetic acid], 1 mM Ca2+ or 1 mM Sr2+.