Abstract: The mechanism of relaxing effects of lidocaie(Lid)on the contraction of isolatedrabbit thoracic aortic rings was studied. Verapamil(Ver)was used for comparison. Lid and Vershowed obvious relaxing effect on contraction evoked by high K⁺ depolarization,In the study ofcontractions induced by noradrenaline(NA) ,both Lid and Ver were shown to suppress the componentelicited by calcium release from the intra-cellular store, but not that induced by calcium influx. Lidshifted the dose-response curves for KCl and NA to the right nonparallelly and depressed their maximalresponses. The inhibition to KCl was more potent than that to NA. This suggests that Lid has selectiveblocking effects on PDC channel , but not the ROC channel. Lid shifted the dose-response curve forCaCl2 to the right nonparallelly and depressed its maximal response. The results indicate that thecalcium antagonistic property of Lid may be one of the mechanisms of its vascular smooth musclerelaxing effect.