Inhibition of Na+-Ca2+ exchange processes in canine cardiac sarcolemmal vesicles by several divalent and trivalent cations has been investigated. The order of cation effectiveness in inhibiting initial rates of Nai+-induced Ca2+ uptake in the presence of 140 mM Nai+ and 20 microM Cao2+ is La3+ greater than Nd3+ greater than Tm3+ approximately Y3+ greater than Cd2+ much greater than Sr2+ greater than Ba2+ approximately Mn2+ much greater than Mg2+. The effectiveness of the divalent ions is related to their ionic crystal radius as compared with that of Ca2+. No such relationship was observed for the trivalent ions, which appeared instead to be more effective the larger their radius. Very low concentrations of trivalent ions ((1-6).10(-7)M) caused slight stimulation of Ca2+-exchange uptake. The trivalent ions also inhibited passive and Nao+-induced Ca2+ efflux from sarcolemmal vesicles, in the same concentration range as that for inhibiting uptake. The divalent ions, however, stimulated Ca2+ efflux, possibly via divalent cation-Ca2+ exchange. These various results suggest that the divalent and trivalent cations interact differently with the exchange apparatus in the sarcolemma.