Abstract Initial magnesium transport in low magnesium Ringer in the frog myocardium consists of at least two systems, a fast system with high capacity with a time constant of 30 seconds and a slow transport system that operates with a time constant of 165 seconds. Both transport systems appear to be electroneutral and can operate either against an electrochemical gradient or with an electrochemical gradient. The fast transport system can transport Mg 2+ in an outward direction at 1.84 p mol cm −2 sec −1; the slow system causes Mg 2+ to be transported outward at 0.05 p mol cm −2 sec −1. Gramicidin S (5 μM) decreases the slow outward transport system to 0.01 p mol cm −2 sec −1 and at concentrations greater than 1 μM inhibits not only slow magnesium outflux in low calcium Ringer but also inhibits magnesium influx during recovery of Mg 2+ in Ringer. Gramicidin S at 5μM decreases Mg 2+ influx from .04 p mol cm −2 sec −1 to 0.01 p mol cm 2 sec −1 indicating that influx and efflux may take place on the same transport system. In the presence of 10 mM Mg 2+ Gramicidin S increases magnesium content. Epinephrine increases magnesium efflux and overcome the inhibition of Mg 2+ efflux by Gramicidin S.