CALCIUM transport by the Na+/Ca+exchanger was measured in plasma membranes vesicles purified from rat brain and in primary rat cortical cell culture. Sodium-loaded vesicles rapidly accumulate Ca2+via Na+/Ca2+exchange (Na+i-dependent Ca2+uptake). Extra-vesicular zinc inhibited Na+/Ca2+exchange as evidenced by a reduction of the initial velocity of Ca2+uptake. Significant inhibition of Ca2+uptake was seen at concentrations of zinc as low as μM. Lineweaver–Burk analysis of the data was consistent with noncompetitive inhibition with respect to extravesicular Ca2+concentration. The Kifor zinc inhibition of Ca2+uptake determined from a Dixon plot was 14. μM. This is within the range of zinc concentrations thought to be obtained extracellularly after excitation. When vesicles were preloaded with Ca2+, extravesicular zinc also inhibited reversal of Na+/Ca2+exchange (Na+o-dependent Ca2+release) although its potency was much less: concentrations of ≥ 3 0μM zinc were required. Zinc inhibition of Ca+release was not Na+dependent. Na+i-dependent calcium uptake by rat cortical cells in primary culture also was inhibited by zinc. The extent of inhibition was similar to that seen for inhibition of Na+i-dependent Ca2+uptake in membrane vesicles, but the potency was less. The results suggest that Ca2+transport by the Na+/Ca2+exchanger is inhibited by concentrations of zinc thought to be attained extracellularly after excitation.