The distribution of myocardial blood flow (MBF) was evaluated in pressure-induced LV hypertrophy of foxhounds. At the early stage of developing hypertrophy, i.e., 3 months after aortic banding (+ 53% LV weight) resting MBF and flow reserve were not significantly different from control hearts. One year after banding (+94% LV weight) myocardial flow reserve had clearly decreased. Acute coronary stenosis of 60 and 70% cross-sectional area resulted in a moderate fall of flow reserve in controls and hearts with early hypertrophy. In hearts with extensive hypertrophy the drop of poststenotic MBF was considerably greater affecting preferentially the subendocardium. The data suggest that myocardial blood flow was impaired before any signs of heart failure were observed. Furthermore a decrease of MBF may not be confined to forms of hypertrophy induced by renal and idiopathic hypertension.