The coronary hemodynamic effects of controlled aortic insufficiency (AI) were studied in 10 dogs. Coronary blood flow (CBF), before and during reactive hyperemia (RH) with graded coronary diameter narrowings (CN), aortic (Ao) and left ventricular (LV) pressures (P), and aortic blood flow (AoF) were recorded. Opening an adjustable basket catheter, positioned across the aortic valve, created reversible AI quantitated from phasic AoF. AI was regulated so that mean CBF was similar with or without AI. During AI, heart rate and systolic AoP were unchanged, but diastolic AoP declined 14 mm Hg (mean) and end-diastolic LVP increased 8 mm Hg, both p less than 0.05. With CN greater than or equal to 85%, mean CBF decreased with or without AI. Coronary resistance was similar with or without AI. During AI with no CN, peak RH CBF declined significantly and was similar to peak RH with 70% CN without AI. Furthermore, AI with 60% CN caused additional reduction in peak RH and was similar to peak RH with 80% CN without AI. These data suggest that CBF reserve, exposed during RH, is decreased during AI. With AI, a given CN has coronary hemodynamic properties similar to higher degrees of CN without AI. These results may relate to clinical findings of ischemia in patients with AI and no or moderate CN.