A method for multiple simultaneous determinations of cardiocirculatory dynamics, regional blood flow, and total cardiac output distribution in the conscious rat preparation is described. The preparation allows for intravenous administration of agents and can be performed on animals either at rest or during treadmill exercise. Instrumentation procedures involve placement of fluid-filled catheters in the left ventricle, right atria, right jugular vein, and caudal artery. Left ventricular pressures are recorded via a modified 4F Millar transducer-tipped manometer containing a 10-cm extension of fluid-filled PE 50 placed into the left ventricle via the right carotid artery. Radionuclide-labeled microspheres (15 +/- 5 mu) are injected into the left ventricle through the fluid-filled PE 50 at selected times for determination of cardiac output and regional blood flows using the caudal artery catheter as the source of the reference blood sample. Details and selected validation data for procedures involving anesthesia, instrumentation, recovery from anesthesia, data gathering, and data analysis are presented. Emphasis is placed on the procedures required for use of the radioactive microsphere technique in this model with special attention given to quality control of the microsphere stock, counting procedures, and computer analysis of these data.