The study of the steady state and transient photoconductivity (PC) of p- and n-type porous silicon (PS) self-supporting layers allows us to eliminate the influence of a crystalline Si substrate on the PC. A photovoltaic effect was observed in sandwich geometry for short wavelength illumination and for long wavelengths (similar or equal to 580 nm) a broad photoconductive maximum was found. An explanation of both effects is proposed. Transient photoconductivity has revealed a redistribution of the inner electric field after 1-10 ms and it is proposed here as a new tool for the study of PS.