The influence of Mg on Ca- and K-related responses of rabbit aortic strips was studied. Aortic strips bathed in either a K-free or 60.0 mM K solution were less sensitive to Ca in the presence of 1.2 mm Mg. Ca induced greater maximum tension in the aortic strips in the presence of Mg when bathed in 60.0 mM K. Addition of K to a K-free medium induced a slight relaxation at low concentrations (is less than 1.0 mm) which was Ca dependent. The relaxation was less in the presence of Mg. Wiolution were less sensitive to Ca in the presence of 1.2 mm Mg. Ca induced greater maximum tension in the aortic strips in the presence of Mg when bathed in 60.0 mm K. Addition of K to a K-free medium induced a slight relaxation at low concentrations (is less thth Mg in the medium throughout the experiment (90 min), the threshold concentration of K, including a contractile response, was increased as was the maximum tension. Acute addition of Mg decreased the maximum response to K. When K was lowered from 5.4 to 0.0 mm a contractile response, absent in the presence of Mg, occurred. When the K was increased, the tension returned to baseline. At 25-30 mm K, the tension again increased, reaching a maximum at 40-50 mm K. Ca uptake was stimulated at all K concentrations producing a contraction. Mg depressed this increase in uptake. 45Ca efflux was more rapid into a 50.0 mm than into 0.0 mm K solution. Mg delayed Ca efflux. It is concluded from these data that Mg has two components in its effect on the Ca associated with K responses. One is a competition with Ca at extracellular sites probably at the membrane, and the other is intracellular, probably a competition with Ca at sequestration sites.