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Yeast can proliferate in environments containing very high Ca²⁺ primarily due to the activity of vacuolar Ca²⁺ transporters Pmc1 and Vcx1. Yeast mutants lacking these transporters fail to grow in high Ca²⁺ environments, but growth can be restored by small increases in environmental Mg²⁺. Low extracellular Mg²⁺ appeared to competitively inhibit novel Ca²⁺ influx pathways and to diminish the concentration of free Ca²⁺ in the cytoplasm, as judged from the luminescence of the photoprotein aequorin. These Mg²⁺-sensitive Ca²⁺ influx pathways persisted in yvc1 cch1 double mutants. Based on mathematical models of the aequorin luminescence traces, we propose the existence in yeast of at least two Ca²⁺ transporters that undergo rapid feedback inhibition in response to elevated cytosolic free Ca²⁺ concentration. Finally, we show that Vcx1 helps return cytosolic Ca²⁺ toward resting levels after shock with high …