A pH- and temperature-sensitive hydrogel of poly(beta-amino ester)-poly(epsilon-caprolactone)-poly (ethylene glycol)-poly(epsilon-caprolactone)-poly(beta-amino ester) (PAE-PCL-PEG-PCL-PAE) pentablock copolymer was evaluated as a sustained injectable insulin delivery system. Insulin was readily loaded into the matrix, forming an ionically linked insulin-PAE complex. Complex mixtures containing various concentrations of insulin and copolymer were subcutaneously injected into male Sprague-Dawley rats to study the profile of insulin release in vivo. The insulin-release profile showed that insulin was maintained at a constant steady-state level for 15 days, and further demonstrated that insulin levels were controlled by the amount of insulin loaded into the copolymer and the copolymer concentration in the hydrogel. The effect of the insulin-gel complex was further investigated in the streptozotocin (STZ) diabetic rat model. After subcutaneously injecting complex mixtures into STZ-induced diabetic rats, blood glucose and plasma insulin levels were measured. The results showed that the diabetic rats could be treated for more than 1 week with a single injection of the complex mixture containing 10 mg/mL insulin in a 30 wt.% copolymer solution, suggesting that this pH/temperature-sensitive insulin-hydrogel complex system may have therapeutic potential.