The effective delivery of angiogenic factors is a useful strategy for the engineering of vascularized tissues. When adrenal cortical cells were implanted in mice under the renal capsule, the size of the implant was reduced to about 100 microm in thickness after 8 weeks. Either low (approximately 2 microg) levels of basic fibroblast growth factor (bFGF) or high (approximately12 microg) levels of bFGF were encapsulated into poly-lactic-co-glycolic acid microspheres, and these bFGF-encapsulated microspheres were coimplanted with adrenal cortical cells. After 56 days, the implants with low and high levels of bFGF weighed five and eight times more, respectively, than the implants without bFGF delivery. The implants with bFGF-encapsulated microspheres also contained significantly more cells than the implants without bFGF delivery. The levels of adrenal cortical gene expression were not significantly changed with bFGF delivery. The implants with high levels of bFGF also had a more uniform distribution of anti-CD31 immunofluorescence. Based on the increased number of cells that expressed adrenal cortical genes, the delivery of bFGF enhanced adrenal cortical cellular regeneration, possibly through an angiogenic response.