This study is designed to examine the effects of basic fibroblast growth factor (bFGF) administration on the biomechanical properties of thin skin flap healing. A total of 42 rats were used in this study, and skin flaps 10 cm long by 3 cm wide were raised in 28 rats. One injection of bFGF was applied at three different times (immediately postoperatively, and 24 h and 48 h postoperatively) between the flap and wound bed of 14 rats (the bFGF treated group), while the other 14 rats with flaps had the same tissue culture medium treatment but without bFGF (the untreated group). The remaining 14 rats without flaps constituted the control group (normal group). The rats were killed 10 days postoperatively, and 1. 0 cm x 6.0 cm sections of the skin flap taken for mechanical and histological testing. The load, deformation and tensile strength at failure were recorded. The average flap survival area in the bFGF treated group was 27 cm(2), significantly higher (P< 0.001) than that seen in the untreated group. The average elastic stiffness of the skin flap in the bFGF treated group was also observed to be higher than in the untreated group although this difference was not significant. The mean tensile strength of the bFGF treated group (61 N) was significantly higher than the untreated group (38 N, P< 0.01) however. Despite this, the tensile strengths at failure of both of these groups were found to be significantly lower than that of normal skin (101 N, P< 0.01). A relaxation in load of about 8% was seen in specimens from the normal group and the bFGF treated group, while the untreated group showed a relaxation of about 15%.