The purpose of the present study was to examine subendocardial flow and function during graded coronary pressure reduction to determine the effect of tachycardia on the lower autoregulatory pressure limit (critical coronary pressure) in unanesthetized dogs. During atrial pacing at a rate of 200 beats/min, subendocardial flow measured by radioactive microspheres averaged 1.55 +/- 0.34 ml/min/g and remained unchanged as pressure was reduced over the autoregulatory plateau from 84 +/- 10 to 59 +/- 7 mm Hg. Further reductions in coronary pressure to below a critical coronary pressure of approximately 60 mm Hg were associated with concomitant reductions in subendocardial flow and the endocardial-epicardial flow ratio during tachycardia. Although regional function remained constant over the autoregulatory plateau, there was a rightward shift of the coronary pressure-function relation during ischemia in response to a steady-state increase in rate from 100 to 200 beats/min. Reductions in regional wall thickening began when coronary pressures reached 38 +/- 7 mm Hg at a heart rate of 100 beats/min and 61 +/- 6 mm Hg at a heart rate of 200 beats/min (p less than 0.005). Similar critical coronary pressure values were obtained for subendocardial segment shortening. Relations between subendocardial flow and myocardial function measured by both transmural wall thickening and subendocardial segment shortening were linear during pacing at a heart rate of 200 beats/min with relative reductions in wall thickening related to reductions in subendocardial flow on a nearly one-to-one basis. The results of this study demonstrate that there is a shift in the lower limit of subendocardial autoregulation during tachycardia as manifest by the onset of subendocardial ischemia at a higher distal coronary artery pressure. The shift in critical coronary pressure relates to an increase in resting flow requirements due to increased demand and diminished subendocardial vasodilator reserve at any given coronary pressure secondary to a reduction in the time available for diastolic subendocardial perfusion during tachycardia.