Sinus node cells in the isolated perfused canine right atrium and cells of the right ventricular false tendons were used to assess the effects of extracellular Mg2+ concentration [Mg2+](o)) on cardiac electrical activity. Removal of Mg2+ from the perfusate into the sinus node led to an increase of 36% in sinus rate that was sustained for as long as Mg2+ was absent; doubling [Mg2+](o) to 2 mmol/liter caused the sinus rate to decrease by 19%. During Mg2+-free perfusion, the accelerated sinus rate could be depressed by the addition of certain substances; verapamil and Mn2+ produced the same percent depression regardless of [Mg2+](o), but tetrodotoxin and reduced [Na+] in the solution each brought about significantly greater depression of sinus rate when Mg2+ was absent. No change in maximum diastolic potential was observed in the sinus node cells when Mg2+ was withheld. In false tendon cells, on the other hand, removal of Mg2+ was accompanied by depolarization of the transmembrane potential to a stable level of approximately -40 mV, at which potential action potentials could not be elicited. Increasing [Mg2+](o) to 4 mmol/liter caused slight hyperpolarization of false tendon cells, but maximum upstroke velocity of the action potential and overshoot were reduced in spite of the more negative resting transmembrane potential. In both types of tissue, sinus node and false tendon, all changes clearly began to reverse within 30 minutes after restoration of normal [Mg2+](o), and recovery was complete by 60 minutes. Since neither atropine, 5 μg/ml, nor propranolol, 15 μg/ml, modified the responses to Mg2+ alteration, these results indicate that Mg2+ has a direct effect on transmembrane electrical processes. The functional response to changes in concentration of this cation depends upon the specific type of cell acted upon.