For any given perfusion pressure the difference between coronary autoregulated and maximally vasodilated flow represents the flow reserve. If hypertension and cardiac hypertrophy are present, the line of autoregulated flow becomes higher, and the pressure-flow relationship at maximal vasodilation less steep, due to the raised resistance. In these circumstances, flow reserve reduces and the point at which rest flow equals maximal achievable flow may be shifted to a higher perfusion pressure. Thus, flow would decline even if the perfusion pressure is lowered to normal. We tested this point in a setting of patients having chest pain and normal angiography of the left epicardial branches. Baseline flow (ml/min) from the great cardiac vein (thermodilution) was 142 +/- 13 in 9 normotensives (controls), 144 +/- 15 in 7 hypertensives (Group 1) with normal (114 +/- 11 g) left ventricular mass index and 188 +/- 17 in 8 hypertensives (Group 2) whose left ventricular mass (171 +/- 24 g) exceeded the mean +2 SD of normal. Coronary perfusion pressure was lowered in these patients by 5 mmHg every 5 minutes with a titrated nitroprusside infusion, taking as endpoints a perfusion pressure of 60 mmHg in the controls and of 70 mmHg in hypertensives. At endpoints, flow was similar to baseline in controls and Group 1. In Group 2 flow started to decline and myocardial oxygen extraction to slightly but significantly rise at perfusion pressure of from 90 to 80 mmHg; at the endpoint flow was reduced by 26% of baseline (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)