Classical models for assessing myocardial function suffer from either a potentially anoxic central core or an unsophisticated perfusion system. We have improved Langer's perfused interventricular septum model by adapting it for parabiotic resuscitation, thus achieving rapid stability and significantly less edema. The new model, which consists of a triangular piece of rabbit interventricular septum, perfused through its septal artery, first undergoes a stabilization period of 30 min during parabiotic perfusion from another rabbit. An arteriovenous shunt is then opened and the preparation remains stable for 2 h of perfusion in which fresh blood is recirculated nonparabiotically. It is proposed that during this period, experimental interventions could be undertaken using each septum as its own control. The behaviour of 66 preparations was consistent with classical concepts of myocardial function.Simple measurements were identified for beat-to-beat functional assessment. Contractility was measured by maximum active tension and maximum differential of tension. Relaxation was measured by maximum negative differential of tension and, in the presence of ischemic contracture, changes in resting tension. Compliance was measured by septal length at a specified resting tension. Perfusability was measured by the relationship between pressure and flow.