In vivo release of basic fibroblast growth factor (bFGF) from a biodegradable gelatin hydrogel carrier was compared with the in vivo degradation of hydrogel. When gelatin hydrogels incorporating 125I-labeled bFGF were implanted into the back subcutis of mice, the bFGF radioactivity remaining decreased with time and the retention period was prolonged with a decrease in the water content of the hydrogels. The lower the water content of 125I-labeled gelatin hydrogels, the faster both the weight of the hydrogels and the gelatin radioactivity remaining decreased with time. The decrement profile of bFGF remaining in hydrogels was correlated with that of hydrogel weight and gelatin radioactivity, irrespective of the water content. Subcutaneous implantation of bFGF-incorporating gelatin hydrogels into the mice induced significant neovascularization. The retention period of neovascularization became longer as the water content of the hydrogels decreased. To study the decrease of activity of bFGF when implanted, bFGF-incorporating hydrogels were placed in diffusion chamber and implanted in the mouse subcutis for certain periods of time. When hydrogels explanted from the mice were again implanted, significant neovascularization was still observed, indicating that most of the biological activity of bFGF was retained in the hydrogels. It was concluded that, in our hydrogel system, biologically active bFGF was released as a result of in vivo degradation of the hydrogel. The release profile was controllable by changing the water content of hydrogels.