The aim of this study was to investigate stability and tissue response to poly (L-lactic acid) (PLLA) membranes implanted in sub-dermal tissue of rats. Membranes with and without plasticizer (triethylcitrate) were compared. Membranes without plasticizer were denser and more compact than those with triethylcitrate. Fifteen days and 30 days after implantation, the membranes with tissue adhered were removed and processed for light microscopy, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). By 15 days post-implantation, membranes lacking plasticizer showed invasion of the pores by connective tissue. Thirty days after implantation, the pores of membranes with plasticizer were invaded by blood vessels, and multi-nucleated giant cells surrounded by globular units of the membranes. Membrane debris was also detected in the cytoplasm of multi-nucleated giant cells. These data show that the addition of plasticizer to PLLA results in a more porous membrane, therefore enabling them more suitable in tissue repair (than membranes without plasticizer).