The effects of chronic pressure overload hypertrophy on myocardial blood flow and capillary density was measured in the feline left ventricle. Myocardial hypertrophy was produced by and 84% banding constriction of the ascending aorta 2.8 +/- 1.2 months before the experiments. In seven cats with aortic constriction, cardiac hypertrophy produced a 40% increase in left ventricular mass. Seven cats served as normals. Our findings show that, in chronic pressure overload hypertrophy, coronary blood flow at control (resting) levels is increased compared with normals. In both normal and hypertrophy cats endocardial/epicardial flow ratios were equal at the control level. In the hypertrophied hearts, coronary reserve, measured as the percentage increase in myocardial blood flow from control to near maximal flow during adenosine infusion, was reduced. In the hypertrophy group a shift in the transmural distribution of blood flow in the left ventricle was noticed, as indicated by a reduced endo/epi flow ratio, during adenosine infusion. A decreased capillary density in hypertrophy, most marked in endocardial tissue regions, was demonstrated by this study. These findings indicate that capillary growth does not parallel myofibre growth in the endocardium of pressure overload hypertrophied left ventricles. The resultant anatomical imbalance causes a compromise of flow reserve in the endocardium, making this region vulnerable to ischaemia.