The purpose of the investigation was to determine the effects of a daily regimen of near-maximal contractions, produced via in vivo electrical stimulation of peripheral nerve, on functional and histochemical properties of rat hindlimb muscles immobilized for 28 days in a plaster cast. Rats had knee and ankle joints of one hindlimb immobilized; then while anesthetized, half of the group was subjected to a daily regimen of 480 semifused tetanic contractions (50 Hz) via fine-wire electrodes chronically implanted around the sciatic nerve. Immobilization caused significant decreases in soleus and gastrocnemius muscle weights, fiber cross-sectional areas, and twitch and tetanic strength measured in situ. In addition, immobilized soleus muscles had faster time to peak tension (TPT) and higher proportions of fast-twitch fibers, whereas immobilized gastrocnemius muscles demonstrated faster half-relaxation times (RT1/2) and total twitch durations (TPT plus RT1/2). The only significant effects of the imposed contractions were evident in the gastrocnemius in which stimulation prevented the shortening of RT1/2 and total twitch duration and resulted in significantly higher relative tensions at 50 Hz and higher fatigue resistance. Muscle activity of this type imposed on immobilized muscle is ineffective in attenuating atrophy but can, in fast muscle such as gastrocnemius, prevent changes in twitch characteristics resulting from immobilization, as well as augment contractile responses during semifused and fatiguing contractions.