One of the key technologies for regeneration of damaged and lost tissue is the sustained release of biologically active growth factors. The present study was undertaken to investigate sorption and desorption of various growth factors from biodegradable hydrogels prepared through glutaraldehyde crosslinking of gelatin with isoelectric points (IEPs) of 5.0 and 9.0, which are named 'acidic' and 'basic' gelatins, respectively, based on their overall charge. Basic bFGF and TGF-β1 were markedly sorbed with time in the acidic gelatin hydrogels, while less sorption took place in the basic gelatin hydrogels. This behavior was explained in terms of an electrostatic interaction between the basic growth factors and the acidic gelatin. However, BMP-2 was sorbed into the acidic gelatin hydrogel to alesser extent than the other two growth factors, even though its IEP is also greater than 7.0. An in vivo experiment revealed that the acidic gelatin hydrogel was degraded with time, while growth factors were retained in the body for a longer time period as the in vitro sorption to the acidic gelatin hydrogel was larger. These findings indicate that the growth factors ionically complexed to the acidic gelatin hydrogel were released in vivo as a result of hydrogel degradation. Furthermore, animal experiments revealed that the biological performance of growth factors was enhanced by their sustained release, in marked contrast to the growth factors administered in the solution form.