Diastolic perfusion time is an important determinant of coronary blood flow and subendocardial perfusion. It has been proposed that subendocardial ischemia may exacerbate and perpetuate left ventricular dysfunction in congestive heart failure. Diastolic perfusion time in relation to heart rate was analyzed in 29 digitalized (group 1) and 12 nondigitalized patients (group 2) with heart failure and in 58 normal control subjects. In group 1 there was a strong negative exponential correlation (r = -0.85) and in group 2 a strong negative logarithmic correlation (r = -0.95) between heart rate and diastolic time; both regressions differed significantly from normal control. There was a 9% increase of diastolic time at a heart rate of 60 bpm in group 1 and a 7% increase in group 2 (both p less than 0.05) compared with normal subjects. The curves intersected the regression line of normal subjects at a heart rate of 98 bpm in group 1 and 93 bpm in group 2. At 120 bpm there was a 10% decrease in diastolic time for both groups with heart failure (both p less than 0.05). Changes in diastolic perfusion time relative to heart rate are more pronounced in congestive heart failure such that at faster heart rates this relationship may further impede subendocardial blood flow.