Intraluminal stenosing cylinders were inserted in the coronary arteries of open-chest, anesthetized dogs to assess the sensitivity of sympathomimetic infusion for detection of subcritical impairment of reactive hyperemia. Observations were made at rest and during steady-state infusions of dopamine and dobutamine (each 10 micrograms/kg/min) before and after placement of the cylinder. Each stenosis was associated with subcritical impairment of postocclusion reactive hyperemia at rest. The degree of impairment was used to stratify experiments into mild (group A) and moderate (group B) cohorts. In group A, reactive hyperemia was 217 +/- 55 cc/min prior to cylinder placement and 82 +/- 17 cc/min (p less than 0.002) after insertion. In group B, reactive hyperemia was 235 +/- 54 cc/min and 63 +/- 7 cc/min (p less than 0.001) before and after insertion. Both drugs resulted in a significant increase in regional shortening (ultrasonic crystal technique) in the absence of a stenosis. After creation of the stenoses, dopamine continued to cause a significant increase in shortening in both groups, whereas this increase was impaired in group B during dobutamine infusion (14.8 +/- 5.9% at rest vs 21.4 +/- 10.3% during infusion, p = NS). Thus, with subcritical lesions in a single vessel, dobutamine infusion was associated with depressed regional function when reactive hyperemia was impaired by more than 80%.