Adult cats were light anesthetized with ethyl ether and heart was removed fastly. cardiac papillary muscle was dissected from heart in organ bath con taining Tyrode solution saturated with 95% O₂+5% CO₂, and prepared papillary muscles were placed in Tyrode solution that was continuously circulated and gassed with 95% O₂+5% CO₂at 32℃. The isolated papillary muscle was stimulated continuously with platinum pin electrode at frequency of 15/min and 90/min by means of electric stimulator and transmembrane action potentials were recorded with microelectrdes on the oscilloscope. The drug used was d-propranolol and its concentration was 0.5, 1.5 and 5.0 mg/L. The results obtained were as follows: 1. D-propranolol increased the threshold voltage of papillary muscle and raised by average of 213.6% of control. 2. D-propranolol had no effect on duration of action potential. 3. Conduction time of isolated papillary muscle was increased by d-propranolol and its effect was prominent at frequency of 90/min. 4. the maximum upstroke velocity was decreased by d-propranolol and its effect was dose-depndent decrease.