Abstract: In order to clarify the physiological mechanism of the resistance to metalaxyl of Phytophthora nicotianae var.nicotianae,the changes of the relative leakage of sensitive,lower-resistant and higher-resistant strains treated with metalaxyl at different concentrations within 540 minutes were studied.The results showed that metalaxyl might damage the membrane of P.nicotianae var.nicotianae,whose permeability increased after metalaxyl treatment; the speed and degree of metalaxyl effect on the membrane permeability of the sensitive strain,ZC002 were similar to those of the lower-resistant strain,10M-006, which increased when metalxyl concentration (0.1~100.0 μg/mL) increased; those of the higher-resistant strain,10M-004 were lowest,which increased when metalxyl concentration (50.0~100.0 μg/mL) increased.10M-004 had stronger ability to adapt to metalaxyl than ZC002 and 10M-006 had,which decreased when metalaxyl concentration increased.