An in situ versus in vitro comparison of relative dose-dependent effects of halothane on cardiac performance was investigated, including ventricular systolic/diastolic function. Such comparative studies may be of interest to individuals working on heart failure models, cardiac device testing, or xenotransplantation. Normal swine (n = 9) received halothane at levels of 0.25, 0.5 and 1 MAC (minimum alveolar concentration) for 30 min each. Parameters assessed included: 1) heart rate; 2) arterial blood pressure; 3) pulmonary artery, central venous, left and right ventricular pressures; 4) cardiac output; 5) end-expiratory CO2 and halothane levels; 6) cardiac temperature; and 7) arterial blood gases. Hearts were removed using standard cardioplegic procedures and reperfused in four-chamber working mode (n = 8); again the effects of increasing halothane concentrations on cardiac performance were analyzed. When comparing biventricular depressive effects (negative inotropic, negative lusitropic) of halothane in vivo and in vitro, there were distinct quantitative differences. The negative lusitropic effects were less pronounced in vitro; this was especially true for the right ventricle. Yet, in vitro, halothane at all doses induced more pronounced decreases in left heart output compared to the right. The large mammalian isolated four-chamber working heart model allows for novel assessment of pharmacodynamics and/or evaluation of cardiac devices under a range of hemodynamic performances. Halothane, a cardiodepressive agent, induced direct myocardial depressive effects in vitro similar to those recorded in vivo; hence additional systemic effects are considered to play a minor role in ultimate performances, e.g., compensatory responses due to autonomic controls.