A copolymer of L- lactic acid (LA) and glycolic acid (GA), which has a relatively low number-average molecular weight ( M ̄ n=1600–3300 ) was synthesized by means of direct copolycondensation without catalysts. The luteinizing hormone-releasing hormone agonist, des-Gly 10-[Leu 6]-LH-RH ethylamide monoacetate (LH-RH agonist), was incorporated by the melt-pressing technique in copoly(LA/GA) to obtain a fine cylindrical formulation. This solid formulation was implanted subcutaneously in the back of male adult Wistar strain rats. The in vivo degradation of copoly(LA/GA) formulation without LH-RH agonist showed a marked acceleration with increasing GA component in the copolymer, and the resulting maximal value was obtained at 30 70 mol% LA/GA composition; for example, the copolymer with M ̄ n=2300 went up to 100% in the 6th week from the implantation. The period required for 100% in vivo degradation increased linearly with increasing M̄ n of the copolymer. From this linear relation it was found that the period required for 100% biodegradation per 1000 g mol increase of M ̄ n corresponds to approximately 6 weeks. The serum LH-RH agonist level in rats in which the copoly( 70 30 mol% LA/GA, M ̄ tn=2900 ) formulation was implanted, which was degraded up to 100% after 16 weeks implantation, was kept constant during the first 10 weeks and then decreased linearly until its level could no longer be detected in the 15th week from the implantation. This finding shows that LH-RH agonist is constantly released in vivo from the biodegradable formulation over a long period.