Myocardial cell pH has been measured with 5,5-dimethyl-2,4-oxazolidinedione (DMO) in intact anesthetized dogs by a transient indicator dilution technique. Bolus injections of labeled DMO, vascular, extracellular, and water indicators were made into the anterior descending coronary artery, and blood samples were collected from the great cardiac vein. The steady-state distribution of DMO between cells and plasma was calculated from the indicator mean transit times, and the plasma pH. Myocardial cell pH was determined from the distribution value and plasma pH. Normal myocardial cell pH averaged 6.94. Changes in myocardial cell pH after infusions of acid or alkali. Myocardial ischemia induced by inflation of a coronary artery balloon resulted in progressive decreases in cellular pH to average values of 6.83 within the initial 15 min and to 6.59 within the interval between 20 and 70 min. Infusions of Na2CO3 tended to diminish intracellular acidosis although these infusions had little effect on the difference in pH between the myocardial cell and extracellular fluid.