The purpose of this study was to determine the isometric twitch tension (Pt), isometric tetanic tension (Po), and twitch contraction time (Ct) of the reinnervating rat soleus and plantaris muscles. Six groups of adult female Wistar rats (200 to 225 gm BW) underwent bilateral sciatic nerve crush at the sciatic notch, while another 6 groups were kept as sham-operated normal controls. The contractile properties of the muscles from the normal and crush-denervated groups (n=6-9) were determined in vitro weekly, at 0, 1, 2, 3, 4, 5, and 6 weeks, post-nerve crush. The muscle were equilibrated in a polyglass cell in oxygenated Ringer's solution at pH 7.2 at 34-35 C. The muscles were field stimulated at optimal length (Lo) to produce maximal Pt, using 15-20 v, 5 amp current, at 0.2 Hz for 1 msec duration. Following twitch determinations, the muscles were tetanized at 200 Hz. All tension values were expressed per unit area of the muscle. The respective control tension per area values for the soleus and plantaris were: Pt, 3.3 +/- 0.4 gm, 5.9 +/- 0.4 gm; Po, 20.7 +/- 5.1 gm, 21.8 +/- 4.8 gm, while the Ct values for the soleus and plantaris were 37.6 +/- 2.1 msec and 16.3 +/- 1.1 msec, respectively. At 3 weeks post-nerve crush, the crush-denervated soleus Pt and Ct increased 40% and 33.6% respectively, and the plantaris 28.2% at 1 week after the nerve was crushed. Gradual recovery was noted in the Pt and Ct from the 4th to 6th weeks post-crush, while the Po began improving 1 to 2 weeks after denervation. The soleus Pt and Po appeared to be more dependent on neural control than the plantaris muscle. During reinnervation, the slow soleus and fast plantaris Ct were equally dependent on innervation. Reinnervation prompted changes in the Po, Pt, and Ct, suggesting that regenerating neurons promote a sequential emergence of normal contractility.