Radionuclide ventriculography was performed at rest and during transoesophageal pacing on nine anaesthetized baboons (Papio ursinus), alternatively with and without administration of a beta-blocker. Left ventricular volumes, cardiac output, and ejection fraction were determined at each heart rate. A gradual decrease in end-diastolic, end-systolic, and stroke volume followed with increasing heart rate, significantly more pronounced under beta-blockade. Cardiac output and ejection fraction showed no significant changes, although tendencies to increase were observed without beta-blockade. Comparisons of these parameter changes were drawn to those previously observed during the first 5 h of septic shock (E. coli). We conclude that the tachycardia present during this phase of shock is not the only contributory factor to cardiac function changes but that other depressant influences prevail as the added effect of the beta-blocker seems to suggest.