We studied 10 dogs to determine if changes in myocardial images could result from alterations in left ventricular (LV) geometry independent of any change in tracer distribution. All dogs received 20 mCi of 99mTc-labeled albumin microspheres (10-45,) via the left atrium, after which control multigated, gated and ungated myocardial perfusion images were obtained. Next, five dogs were sequentially imaged 1) during partial aortic occlusion to increase LV volume; 2) after release of the aortic occlusion; 3) after ligation of the coronary vasculature of the apex of the LV to produce regional dyskinesis; and 4) after nitroprusside induced hypotension to decrease ventricular volume. Only ligation and nitroprusside studies were done in the remaining five dogs. We gave 1.5 mCi of thallium-201 intravenously to two other dogs and obtained sequential ungated control, ligation, and nitroprusside images. At postmortem exam, tracer activity was equivalent at the apex and base for both microspheres and thallium. Nevertheless, partial aortic occlusion caused defects in the apex and anterior wall in four of five dogs. Coronary ligation caused new defects in eight of 10 dogs given microspheres and in both dogs given thallium. With nitroprusside, microsphere and thallium defects generally decreased in size. Gated images showed that defects caused by ligation tended to be larger in end-systole than in end-diastole, while all partial aortic occlusion-induced defects were equal or greater in size in end-diastole compared with end-systole. These results indicate that changes in LV geometry or contraction pattern may cause defects on myocardial images without any change in tracer distribution.