The human ventricular transmembrane action potential has been studied in small pieces of left ventricular papillary muscle removed from patients with rheumatic mitral valve disease at operation. The recordings obtained by a microelectrode technique have been compared with those obtained from normal canine papillary muscle. The findings are considered in the light of the disturbances in the environment of the human cardiac tissue produced by electrolyte disturbances and digoxin treatment. The human ventricular transmembrane action potential is, in general, similar to that of the normal dog. However, the rate of depolarisation and repolarisation of the human tissue is slower than that of the dog. Electrolyte disturbances do not seem sufficient to account for these changes but they may be due to the effect of digitalis treatment or the result of long-standing cardiac disease. A true species difference cannot be excluded.