To assess coronary vasodilator restore after orthotopic heart transplantation, regional myocardial perfusion was measured with oxygen-15-labeled water and dynamic positron emission tomography in 14 cardiac allograft recipients who were not experiencing rejection and who had no angiographic evidence of epicardial coronary sclerosis 15 to 73 months (mean ± SD 43 ± 19) after transplantation (group I). Twelve normal men with an average age of 31 years (group II) served as a control group. Regional perfusion was measured at rest and after the intravenous administration of 0.6 mg/kg body weight of dipyridamole. Rest regional myocardial blood flow was homogeneously distributed throughout the left ventricle and was significantly higher in transplant recipients (mean 1.16 ± 0.26 ml/g per min [range 0.8 to 1.73] than in normal subjects mean 0.85 ± 0.13 ml/g per min [range 0.57 to 0.99); p = 0.001) as was rest heart rate-systoll blood pressure product (rate-pressure product 11,255 ± 2,540 vs. 7,073 ± 1,306; p < 0.001). After dipyridamole, perfusion in the transplant recipients was homogeneous and slightly lower (2.73 ±1.03 vs. 3.40 ± 1.09 ml/g per min; p = NS), whereas rate-pressure product was slightly higher (12,179 ± 2,266 vs. 10,885 ± 1,895: p = NS) than the value in normal subjects. Dipyridamole vasodilator response (dipyridamole/rest myocardial blood flow) ranged from 1.23 to 4.92 (mean 2.50 ± 1.13) in group I and from 2.65 to 5.45 (3.97 ± 0.89) in group II (p = 0.001). Rate-pressure product change (dipyridammole/rest rate-pressure product) was markedly attenuated in transplant recipients compred with the value in normal subjects (1.09 ± 0.09 vs. 1.56 ± 0.27 ml/g/per min; p < 0,001). Normalization for the difference in rest rate-pressure product for each individual subject showed that the adjusted rest show values (group 11.19 ± 0.27, group II 1.39 ± 0.31 ml/g per min; p = NS) and the dipyridamole-vasodilator response data (group I 2.45 ± 0.64, group II 2.47 ± 1.23, p = NS) in the two groups were not significantly different. Thus, in transplant recipients who are not experiencing rejection, the respons iveness of the coronary vasculature to dipyridamole-mediated vasodilation is uniformly preserved among all regions of the left ventricular myocardium.