To study the effects of acute coronary hypotension on the working dog heart in situ, both coronary arteries were cannulated and perfused with oxygenated blood at controlled pressures (40 to 120 mm Hg). At a perfusion pressure of 120 mm Hg, total coronary artery flow appeared to be sufficient (0.95+/-0.08 ml/min-g) to maintain normal cardiac performance for a 2.5-hour observation period. During incremental decreases in coronary perfusion pressure, significant linear correlations were found between coronary flow and cardiac index (r=0.84), left ventricular maximum dP/dt (r=0.83), stroke index (r=0.82), stroke work (r=0.83) and mean arterial pressure (r=0.62). During simulated shock conditions (systolic arterial pressure, less than 75 mm Hg), relative reductions in coronary flow (-60.9+/-4.0%) paralleled changes seen in cardiac function and persisted for 28+/-4 min.