SUMMARY1. The effects of ajmaline, cocaine, phenytoin, lignocaine, procainamide, propranolol, quinidine and trimetazidine on the contractility and the refractory period of isolated rat papillary muscles were compared.2. At the higher concentrations used, all drugs suppressed the contractile tension of the rat papillary muscles.3. The effects of these drugs on the refractory period were compared at concentrations at which the contractile tension was suppressed by 30% of the control. At this point, lignocaine, procainamide, ajmaline and quinidine prolonged the refractory period more than two‐fold while the effects of propranolol and phenytoin were slight, cocaine and trimetazidine had little or no effec