1. Experiments were undertaken on strips of human atrial and ventricular muscle, on right ventricular rabbit papillary muscles and on the rabbit interventricular septum.
2. In all preparations an increase of developed tension after 30 min exposure to ouabain was apparent only at or above a concentration of 10−7 mol/l (73 μg/l).
3. In the rabbit septum there was a net loss of potassium on exposure to ouabain if the concentration was greater than 10−7 mol/l. There was no net loss, or gain, of potassium at lower concentrations. Loss of potassium, indicating inhibition of the sodium pump, could not be dissociated from the positive inotropic effect of ouabain.
4. The greater sensitivity to ouabain of human myocardium in vivo (10−8 mol/l) than in vitro (10−7 mol/l) may be due to cellular changes occurring in isolated preparations.