Chronic electrostimulation of the sacral roots induces an increase in urethral resistance due to activation of muscles of the striated urethral sphincter. Histochemical study of the urethral striated musculature (nonstimulated) revealed 3 main varieties of fibers: 1) Slow twitch fibers (35 per cent of the whole muscle); 2) Fast twitch fatiguable fibers (52 per cent); and 3) An intermediate type, fast twitch fatigue resistant (13 per cent). Ten dogs with chronically implanted electrodes at the 2nd sacral root were subjected to a program of prolonged stimulation under anesthesia. After prolonged electrostimulation, we detected hypertrophy of the striated muscle fibers of the urethra, anal sphincter and stimulated side of the tail. The stimulated muscle fibers showed a higher overall oxidative activity than the controls. This occurred both in intact and spinalized animals. We thus expect the urethral striated musculature to be more resistant to fatigue because of the increased oxidative activity. Stimulated fibers also had increased glycolytic activity as shown by the enhanced intermyofibrillar deposition, especially in the fast twitch fibers. The increased glycolytic activity may also increase fatigue resistance by producing energy during periods of low oxygen supply at the peak of muscular contraction. As a consequence of increase in oxidative and glycolytic capacities and muscular hypertrophy, we expect that the striated musculature of the urethra will be not only more resistant to fatigue but also capable of generating higher tension. Both are important in achieving continence via electrostimulation of sacral nerve roots. Approximately 3 months after conclusion of the stimulation program these changes had gradually reverted to the normal prestimulation level.