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1. The exchange of cellular Mg with external ²⁸Mg in the rat left ventricle was measured in vivo and, under conditions approximating a steady state, in an isolated, working rat heart perfused and contracting at 36-38° C.

2. About 98% of cellular Mg exchanged at a single rate.

3. The rate of exchange in vivo was the same as that observed in independent in vitro measurements of the influx and efflux at the physiological external Mg²⁺ concentration of 0·56 mM. The rate was 0·15 ± 0·02 m-mole/(kg dry ventricle.min) or 0·21 ± 0·02 p-mole/(cm².sec).

4. In the perfused heart the dependence of the influx on the external Mg concentration was hyperbolic with an apparent V_max of 0·31 ± 0·04 m-moles/(kg dry weight.min) and an apparent K_m of 0·57 ± 0·08 mM.

5. The Mg efflux into a solution containing 2·8 mM-Mg was markedly faster than that into a Mg-free solution.

6. These results are interpreted as consistent with a carrier-mediated transport of Mg across the plasma membrane.

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