The shunting, release and transmural distribution of 9 and 15 .mu. radioactive microspheres were compared in isolated hearts and in intact hearts [in dogs] with and without ATP vasodilation. ATP vasodilation caused increase shunting of 9 .mu. spheres but not 15 .mu. spheres (P < 0.05). The transmural distribution of 9 and 15 .mu. spheres in left ventricle and septum were significantly different (P < 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 appears to be significant differences in regional shunting and distribution in the intact heart. No systematic correction for comparing flows measured with 9 and 15 .mu. spheres were found.