SUMMARY The cardiac output fall and the rate changes observed with halothane-oxygen anaesthesia represent a homeostatic adjustment to greater oxygen extraction per unit volume of blood, secondary to 98 per cent oxygen breathing and to a decreased oxygen demand of the anaesthetized patient. The ability of the heart to restore the pumping function toward control levels by rate change alone strongly challenges the concept that clinical concentrations of halothane expose the myocardium to undue degrees of direct depression. The favourable clinical impressions of clinicians can be reconciled with the findings of the investigators.