Mechanical function and tissue pH (pH t) were recorded in the isolated interventricular septum of the rabbit heart during and after hypoxic perfusion. The perfusate contained glucose and was equilibrated with 95% N 2 and 5% CO 2. The results were compared with those obtained during and after total ischaemia. pH t began to fall after 2 min hypoxia, the overall decline being 0.52 ± 0.11 units after 60 min. Developed tension declined more gradually in hypoxia than ischaemia, but the recovery after hypoxia was both slower and less complete. pH t recovered more slowly after hypoxia than ischaemia. The results suggest that acidosis is a more important determinant of changes in contractile function in hypoxia than has hitherto been recognized.