The mechanism of relaxing effects of lidocaine (Lid) on the contraction of isolated rabbit thoracic aortic rings was studied. Verapamil (Ver) was used for comparison. Lid and Ver showed obvious relaxing effect on contraction evoked by high K+ depolarization. In the study of contractions induced by noradrenaline (NA), both Lid and Ver were shown to suppress the component elicited by calcium release from the intra-cellular store, but not that induced by calcium influx. Lid shifted the dose-response curves for KCl and NA to the right nonparallelly and depressed their maximal responses. The inhibition to KCl was more potent than that to NA. This suggests that Lid has selective blocking effects on PDC channel, but not the ROC channel. Lid shifted the dose-response curve for CaCl2 to the right nonparallelly and depressed its maximal response. The results indicate that the calcium antagonistic property of Lid may be one of the mechanisms of its vascular smooth muscle relaxing effect.