Parenteral delivery systems for proteins and peptides based on aliphatic polyesters are currently the subject of intensive research efforts. Linear polyesters of lactic acid and glycolic acid present significant problems with respect to the modulation of release properties of higher molecular weight substances, such as proteins. The molecular architecture of poly(lactic acid) (PLA) and its copolymers with glycolic acid (PLG), can be modified by the introduction of a hydrophilic charge-containing backbone, e.g. dextran sulfate sodium (DSS) or diethylaminoethyl dextran chloride (DEAED). The resulting branched molecular structure should offer additional possibilities to manipulate the degradation and release properties of parenteral delivery systems. The degradation of the graft PLG is accelerated significantly by the nonlinear structure, which contains many short biodegradable branches attached to a hydrophilic backbone molecule. The mechanism of the polymer degradation is influenced by the backbone substances. In the case of DEAED as backbone, the predominant chain scission of the graft polyester occurs in a random hydrolytic ester cleavage, similar to PLG. By contrast, a nonrandom chain scission in the vicinity of the branching points of the backbone was found for DSS-PLG. The erosion of the graft PLG proceeds more rapidly in the centre of the devices than at the surface. In contrast to linear PLG, the release of FITC-dextran and bovine serum albumin (BSA) from the microspheres prepared from the graft PLG is continuous. Graft PLGs offer additional possibilities for adjusting the release of proteins and peptides from biodegradable parenteral delivery systems.