The shunting, release, and transmural distribution of 9 and 15 mu radioactive microspheres were compared in isolated hearts and intact hearts with and without adenosine triphosphate (ATP) vasodilation. ATP vasodilation caused increase shunting of 9 mu spheres but not 15 mu spheres (p less than 0.05). The transmural distribution of 9 and 15 mu spheres in left ventricle and septum were significantly different (p less than 0.01) and the difference was characteristic for the isolated heart and the nonvasodilated intact heart. There was no difference in distribution across the right ventricle. There appear to be significant differences in regional shunting as well as distribution in the intact heart. We found no systematic correction for comparing flows measured with 9 and 15 mu spheres.