Amorphous copoly( l-lactide)/glycolide, 70 30 mol%) with weight average molecular weights of 16 900–41 300 were synthesized by ring-opening polymerization in the presence of catalysts using a molecular weight moderator lauryl alcohol. The in vivo degradation profiles of the copolyesters, which were evaluated by implanting them subcutaneously in the back of rats, showed a typical S-type degradation pattern. A luteinizing hormone-releasing hormone agonist (LH-RH agonist), des-Gly 10-[Leu 6]-LH-RH ethylamide monoacetate, was incorporated into the small cylinders of copoly ( l-lactide/glycolide) with a weight average molecular weight of 24 000. The cumulative amount of drug released in vivo from the cylinders showed an S-type profile in analogy with the in vivo degradation pattern. This was demonstrated from data such as serum drug level and pharmacological influence on rat prostates.