Growth-hormone-releasing peptide 6 (GHRP-6) plays an important role in animal growth. However, there have been few studies focusing on the effect of GHRP-6 on animal growth through controlled release systems. We synthesized the poly(lactic-co-glycolic acid) (PLGA)–poly(ethylene glycol) (PEG)–PLGA copolymer to investigate its controlled released effect on GHRP-6 in vitro and to study the effect of a GHRP-6–copolymer hydrogel on the growth of rex rabbits. The copolymer was synthesized with ring-opening copolymerization and characterized by 1H-NMR. The interaction between GHRP-6 and the copolymer was characterized by Fourier transform infrared spectroscopy and X-ray diffraction. The body weight, serum level of insulin-like growth factor 1 (IGF-1), and hair coat quality were studied in rex rabbits. The results show that hydrogen bonds formed between the NH group in GHRP-6 and the CO group in the copolymer. The release mechanism of GHRP-6 was a combination of a diffusion-controlled mechanism and an erosion-controlled mechanism in the copolymer. The serum level of IGF-1, hair coat quality, and body weight were all significantly higher in the GHRP-6–copolymer hydrogel group than in the other groups. These results indicate that the copolymer effectively controlled the release of GHRP-6. In addition, the GHRP-6–copolymer hydrogel increased the synthesis of IGF-1 for a prolonged period and, thereby, increased the rex rabbits' growth and hair coat quality. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40185.