We have measured and analyzed the lime-dependent variation in the ac impedance of lipid-impregnated polycarbonate membrane filters (PCMs). The results showed that the electrical characteristics of PCMs gradually changed with time. From the results, we inferred the formation of bilayer-like micromembranes within the PCM pores and presented a model of the time-dependent thinning process of a lipid + decane micromembrane system. In addition, we measured the time-dependent variation of the photoresponse of purple membrane reconstituted onto the PCM system and showed that thinning : of the micromembrane proceeded in all the PCM pores. The results provided information about what is occurring within lipid-impregnated PCMs. Hence such filters will help us explore model systems simulating certain biological membrane systems. The time-dependent variations in the characteristics of such lipid-impregnated PCM systems were previously unknown, although these systems are accepted as stable and feasible candidates for simulating biological functions.