Cryothermic ablation of myocardium and portions of the specialized cardiac conduction tissue has been employed successfully for the treatment of both supraventricular and ventricular tachyarrhythmias. As more uses have been found for cardiac cryosurgery, increasing flexibility of the cryothermia system has been required. The present study was designed to develop a method for electively increasing or decreasing the dimensions and volume of cryolesions by altering adjacent myocardial temperatures. Fourteen dogs were subjected to cardiopulmonary bypass, and standard cryothermic exposures (4 mm cryoprobe, - 60 °C for 120 seconds) were used to create cryolesions on the left ventricular free wall. The dimensions and volumes of cryolesions created at myocardial temperatures of 37 °, 32 °, and 6 ° to 12 °C (hypothermic cardioplegic arrest) were compared. Cryolesions created at 6 ° to 12 ° C with the dogs under cardioplegic arrest were significantly larger (P < 0.05) than cryolesions created at 37 ° or 32 °C in the perfused beating heart. This information may be useful in improving the results of myocardial cryoablation, particularly in procedures requiring the ablation of large regions of myocardium or regions deep below the epicardial or endocardial surface. © 1983.