Vesicles are suspended in a lipophilic ion solution and the lipophilic ion concentration is changed by the permeation of the lipophilic ions into the vesicles. An electrode selectively responsive to the lipophilic ions is immersed in the suspension to detect the time-change rate of the electrode potential, thereby to obtain a change in the lipophilic ion concentration with respect to time, from which are measured membrane characteristics of the vesicles, such as the permeability of a vesicular membrane to the lipophilic ions.