Decreases in regional endocardia] function (ultrasonic dimension technique) and blood flow (radioactive microsphere technique) were correlated in 14 conscious dogs with acute graded levels of coronary stenosis. Coronary stenosis affected overall ventricular function only slightly, but induced gradual reductions in regional blood flow (BF) and segment length (SL) shortening in the ischemic zone. The relationship was best fit by an exponential function relating % change in SL to % change in BF; i.e., SL(%Δ) = -161.6e−0.047BF(%Δ) (r= 0.92). In 14 segments, where no change in function was observed, blood flow fell by 6 ± 1%. However, 10-20% reductions of blood flow impaired function significantly. Severe reduction of blood flow was required to reduce function completely. In 12 segments exhibiting paradoxical motion, blood flow fell by 95 ± 2%, a value greater(P< 0.01) than in four akinetic segments, i.e., those with no shortening or lengthening (−82 ± 4%). These data, which show on one hand that only 10-20% reductions in blood flow impair function significantly and that, on the other hand, severe reductions in blood flow are required to abolish active shortening completely, indicate sensitive coupling between blood flow and function in the conscious dog with acute myocardial ischemia.Circ Res 47: 201-207, 1980