Development of an index of myocardial contractility that is both load independent and easily quantifiedin vivohas been a difficult task. Recently, three measures of contractile state have been advocated that appear to fulfill these requirements: the end-systolic pressure-length relationship (ESPLR), the ESPLR area, and regional preload recruitable stroke work (PRSW). Because the effects of halo-thane and isoflurane on these indices of contractility have yet to be studied, the purpose of this investigation was to compare the effects of these volatile anesthetics on contractile function as evaluatedviathese techniques in chronically instrumented dogs. Because autonomic nervous system tone substantially influences systemic hemodynamicsin vivo, experiments were performed in the presence of pharmacologic blockade of the autonomic nervous system. Four groups comprised the 36 experiments that were performed with nine dogs. Following inhalational induction, the dogs were maintained on 1.5 MAC and 2 MAC of halothane or isoflurane. Pressure-length loops were generated after 1 h of equilibration using preload reductionviapartial inferior vena caval occlusion or afterload augmentation by a phenylephrine infusion. The PRSW and ESPLR were then calculated, respectively. Slope and length intercept variables obtained from the ESPLR failed to significantly change from control with increasing levels of anesthetic depth despite substantial decreases in other indices of contractility. However, combination of slope and length intercept parameters into the ESPLR area model proved to be a sensitive and easily calculable measure of depressed myocardial function. Similarly, regional PRSW slope precisely reflected changes in contractile state when halothane (62 ± 10 for control to 30 ± 6 erg · cm−2· 10−3· mm−1at 2 MAC) or isoflurane (83 ± 14 for control to 55 ± 8 erg · cm−2· 10−1· mm−1at 2 MAC) were administered. The PRSW slope also demonstrated a significant difference in depressed contractility when equianesthetic concentrations of halothane and isoflurane were compared (63 ± 7% of control with halothane versus 86 ± 4% of control with isoflurane at 1.5 MAC; 50 ± 5% of control with halothaneversus70 ± 6% of control with isoflurane at 2 MAC). The ESPLR area also accurately demonstrated the differential depression in contractile function suggested by recentin vitrostudies when equianesthetic doses of halothane and isoflurane were comparedin vivo.Therefore, while ESPLR slope and length intercept variables fail as indices of myocardial contractility, ESPLR area and regional PRSW slope were shown to be useful indicators of contractile state in the conscious and anesthetized dog.