Poly(dl-lactide-co-caprolactone) was dissolved in N-methyl-2-pyrolidone (NMP) for preparing blank group formulation, and drug group was prepared by dissolving testosterone undecanoate (TU) as model drug in blank group. These formulations rapidly gelled by solvent diffusion effect after subcutaneous injection. The in vivo degradation of copolymer gel in rabbits was investigated after injection. The GPC and H-1 NMR results showed that no matter the gel contained drug or not, it had no effect on degradation rate for the P(DLLA-co-CL) carrier. Degradation products of copolymer were absorbed or excreted based on metabolism. Copolymer molar composition changed slightly in 3 months. The structures and characteristic of copolymers were characterized via DSC, TGA, and SEM, respectively. Experimental results showed that the copolymer had excellent heat resistance. Crystallinity increased gradually during degradation process. A dense cross-sectional structure formed after 90 days. In addition, it can be obviously found that the degradation process of the copolymer proceeded in two steps. Mechanism of copolymer biodegradation in vivo was bulk degradation. The gel could well administrate the release of TU in a sustained way without significant burst features. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3800-3808, 2013