The effects of IABP on the contractile state of the in situ canine heart was evaluated under control conditions and conditions of regional ischemia by determining the transient slope of the end-systolic pressure-volume line, T-Emax. For comparison, the conventional index reflecting contractility, dP/dtmax and other related hemodynamic parameters were also assessed. The results showed that, T-Emax, rather than dP/dtmax, reflected more appropriately the changes in contractile state during activation of the IABP under both conditions. It suggests that IABP has a positive inotropic influence only in the ischemic-state and this is a positive inotropic influence only in the ischemic-state and this is a result of the improvement in the myocardial oxygen supply and demand relation. In addition, the limitations of the new indicator, T-Emax, is discussed.