The nature of reinnervation of fast- and slow-twitch skeletal muscles, as assessed by contractile characteristics, was determined in a rat sciatic- nerve graft model. The isometric contractile function of the fast-twitch plantaris and slow-twitch soleus muscles from hindlimbs of adult male Lewis rats (225 to 250 g) was assessed at 16 weeks after sciatic-nerve grafting. A 3-cm interposition sciatic-nerve graft was performed in the following groups: fresh syngeneic (n = 10), fresh allogeneic (n = 11), 3-week stored allogeneic (n = 9), and freeze-thawed allogeneic (n = 9). A control group consisted of 8 normal unoperated rats. Contractile properties were assessed by stimulating the muscles indirectly via the sciatic nerve. At 16 weeks, soleus and plantaris muscle masses were 40 and 52 percent of controls, while their respective absolute tetanic forces (N) were less than 65 and 45 percent of controls. Analysis of time-dependent contractile parameters showed that the soleus/plantaris ratios for time to peak tension (TTP) and maximum rate of force development (df/dt) were not significantly altered following grafting. However, the ratio for half relaxation time (1/2RT) was significantly reduced from 4.44 ± 0.62 toward a value of 1 following grafting. In this study, the authors found that, when fast and slow muscles were reinnervated from a common nerve, maintenance of differences in rate of force development supported selective reinnervation, while loss of differences in time of force relaxation supported random reinnervation.