A technique for controlled release of liposomes using a porous membrane is presented. The system used is a membrane bound reservoir-type controlled release device. In the reservoir the liposomes exist in an aqueous solution. The reservoir is bounded by a porous membrane, the pores of which are filled with an aqueous solution. The rate of release of the liposome from the reservoir is controlled by the membrane. This technique is demonstrated using 35 nm liposomes prepared from dipalmitoylphosphatidylcholine (DPPC) and the 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) headgroup-labeled dipalmitoylphosphatidylethanolamine (NBD-PE) in a 7:1 ratio. The rate of release was controlled by a polycarbonate track etched screen membrane (PCTE) having an 18 mu m pore size. A constant release rate was achieved for 30 h.