Abstract: Pharmaceutical compositions that enable the release of a physiologically active substance over a prolonged period of time following administration to a patient are described. The pharmaceutical compositions are provided by encapsulation of a physiologically active substance into a matrix comprising biodegradable polymers and lipids. The rate of release of the physiologically active substance from the pharmaceutical composition is controlled by varying the ratio of the polymer to the lipid. The compositions can be stored in an aqueous suspension or as a solid dosage form. The physiologically active substances include small molecules, peptides, proteins, nucleic acids and vaccines. The biodegradable polymers include homopolymers, or random or block copolymers. The lipids include phospholipids, cholesterol and glycerides.