Preservation of left ventricular function with various potassium-based cardioplegic solutions has been considered to be effective for at least 60 minutes during occlusion of the ascending aorta. The purpose of this study was to define the limits of protection offered by potassium alone. A single bolus of 150 ml of potassium (24 mEq per liter) in normal saline solution at 30 degrees C was injected in the aortic roots of foxhounds at the initiation of periods of 45 minutes, 60 minutes, and 75 minutes of aortic occlusion at a core temperature of 30 degrees C. Data derived from postischemic recovery phase ventricular function curves and force-velocity relations demonstrated excellent protection during 45 minutes of ischemia, inconsistent protection at 60 minutes, and poor protection at 75 minutes.