Abstract: dl-tetrahydropalmatine (THP) is an alkaloid isolated from corydalis turtschaninovii Bess. f. Yanhusuo Y. H. Chow et C. C. Hsu. It possesses analgesic, sedative-hypnotic, hypotensive and anti-arrhythmic actions. In order to elucidate its mechanism of anti-arrhythmic action this paper reports its negative inotropic effects, in comparison with verapamil its effects on the dose-response curves of isoproterenol (Iso) and calcium and on the frequency-force relationship in the left auricle of guinea pig.THP possesses dose-dependent negative inotropic effects on the left auricle of guinea pig. The potency of THP to that of verapamil was 1:89. Similar to verapamil, but different from propranolol. THP shifted the isoprenaline dose-response curve non-parallelly to the right with decrease of its maximum effect. THP and verapamil shifted the calcium dose-response curve non-parallelly to the right and decreased their maximum effect. THP produced frequency-dependent depression of contractile of the left auircle of guinea pig. In the presence of THP and verapamil, the depression of the contractile force was more pronounced at high than at low frequencies. It is concluded that the effects of THP on left auricle of guinea pig is similar to that of verapamil, and is antagonistic to Ca²⁺.