Advanced characterization of perovskite solar cells is already in progress yielding highly significant data. However, studies of isolated perovskite thin films under the working conditions of solar cells are still scarce. Presented here are photoconductivity measurements of CH3NH3PbI3 deposited between two dielectric-protected Au electrodes spaced ~2000 nm apart. The photo response of the CH3NH3PbI3 which is subjected to a dc bias, involves two time constants one of which is extremely long, lasting for several seconds. Similar time scale is observed upon transformation back to dark. Our findings seem to clarify the origin of the well-known hysteresis in perovskite solar cells.