Condensation of Bu2Sn(OMe)(2) with poly(ethylene glycol)s yielded macrocyclic tin alkoxides which were, in turn, used as cyclic initiators for the ring-expansion polymerization of E-caprolactone, D,L-lactide, or trimethylene carbonate. The resulting cyclic triblock copolymers were in situ cross-linked with trimesoyl chloride. The lengths of the A-B-A triblock segments were varied via the monomer-initiator ratio (M/I) or via the lengths of the poly(ethylene glycol)s. After extraction with CH2Cl2, the isolated networks were characterized by H-1 NMR spectroscopy, DSC measurements, and swelling experiments. The release of dexamethasone and 5-fluorouracil from two triblock networks was studied in physiological buffer solutions at 37 degreesC over a period of several weeks. A strong initial burst was found in all cases. Only a weak initial burst and a more continuous release was observed when networks of random L-lactide/epsilon-caprolactone copolymers were studied under the same conditions.