Abstract: ZnSO₄ was found to inhibit the contractions of isolated guinea pig aortic preparations induced by KCl, CaCl₂ and noradrenaline (NE). ZnSO₄ (0.01～0.1 mmol/L) shifted the dose-response curves for KCl, CaCl₂ and NE to the right, and depressed their maximal responses with PD₂' values of 3.21, 3.07 and 2.91 respectively. ZnSO₄(0.1mmol/L) significantly antagonized KCl, CaCl₂ and NE-induced contraction responses in isolated dog portal vein preparations. These results suggest that the ZnSO₄ induced relaxation of vascular smooth muscle might result from its antagonistic effect on transmembrane Ca²⁺ channels.