The effect of dinitramine, a selective herbicide, on the plasma membrane of the soybean (Glycine max L.) root was studied. Used as marker systems to observe the herbicide effect were two plasma-membrane-specific enzymes, pH 6.5 ATPase and glucan synthetase.The activity of pH 6.5 ATPase decreased significantly in membrane vesicles prepared from roots harvested 15 minutes after treatment with dinitramine. Maximum inhibition occurred in roots harvested 2 hours after treatment. Glucan synthetase activity decreased similarly within 2 hours of treatment.Membrane permeability to (86)Rb was rapidly increased by dinitramine.The activity of pH 6.5 ATPase returned to the control level within 8 hours of treatment with dinitramine.These results show dinitramine's initial site of action to be the plasma membrane, producing an over-all reduction in membrane function through inactivation of membrane-associated proteins.