To study the effects of calcium in cardioplegic solutions, an in situ dog heart model was used that allowed infusion of two different cardioplegic solutions into separate regions of the same heart. Two concentrations of ionized calcium, 1.0 mM and 0.5 mM, in a cold, potassium-containing solution were tested in two groups of dogs and compared with the same cold, potassium-containing solution but without the calcium, during 100 minutes of global myocardial ischemia induced by aortic clamping. Results were evaluated in terms of percent change of regional systolic shortening measured with ultrasonic piezoelectric crystals, percent change of regional myocardial blood flow, and change of regional left ventricular myocardial diastolic distensibility. No significant differences were found between myocardial regions protected with calcium of either concentration and regions protected with calcium of either concentration and regions protected without calcium. This study could demonstrate no beneficial or adverse effects of including calcium in this type of crystalloid cardioplegic solution applied to an in situ dog heart model.