When coronary sinus and arterial plasma samples obtained during a period of ventricular tachycardia 8 to 11 hours after coronary ligation were analyzed for electrolyte concentrations, it was found that there was an increase in K and Mg and a decrease in Ca and Na. By comparison, when blood samples were taken from sham dogs 8 to 11 hours after the ligatures had simply been placed under the artery, no significant change in plasma cation concentrations over control values was observed. Infarcted ventricular tissue, as compared with noninfarcted tissue, contained an average of 5.1 mEq./100 Gm. wet weight more Na, 4.0 mEq./100 Gm. wet weight less K, and 1.12 mEq./100 Gm. wet weight less Mg.