In the setting of chronic coronary stenoses, percent wall thickening (%WT) both at rest and during catecholamine stimulation can be abnormal despite normal resting myocardial blood flow (MBF). We hypothesized that this phenomenon is related to abnormal MBF reserve. Accordingly, 15 dogs were studied between 7 and 10 days after placement of Ameroid constrictors around the proximal coronary arteries and their major branches, at a time when collateral development had not yet occurred. %WT and MBF were measured at rest, after 0.56 mg/kg of dipyridamole, and at incremental doses of dobutamine (5-40 µg · kg¹ · min¹). Resting %WT and MBF were normal in all four sham dogs. Resting transmural MBF was normal in all segments in the 11 study dogs, despite reduced (2 SD of normal) %WT (<30%) in 40 of 82 segments. MBF reserve was reduced (<3) in segments with reduced %WT, and a close coupling was noted between resting %WT and MBF reserve. All segments showed an increase in %WT with dobutamine up to a dose of 20 µg · kg¹ · min¹, above which those with abnormal endocardial MBF reserve showed a "biphasic" response. It is concluded that, in the presence of chronic coronary stenoses, abnormalities in resting %WT as well as inducible reduction in %WT during pharmacological stress are related to the degree of abnormal MBF reserve.