An experimental study of the influence of the therapeutic ultrasound on the regeneration of the sciatic nerve submitted to a controlled crush injury was carried out in rats. Twenty female Wistar rats weighing 250 g on average were used and divided into two groups of 10 animals each, respectively, submitted to: (1) crush injury followed by ultrasound irradiation and (2) crush injury only. Under general anaesthesia the sciatic nerve was exposed on the right thigh and crushed with a device especially developed and built for this purpose, with a 15,000g constant load for 10 min, affecting a 5mm-long segment of the nerve proximal to its bifurcation. Pulsed ultrasound irradiation (1:5, 1 MHz, 0.4 W/cm(2), 2 min duration) was started the day after the operation and repeated for 10 consecutive days. The sciatic functional index (SFI) was evaluated at weekly intervals up to the third week, when the animal was killed for histologic and nerve fiber density studies of the sciatic nerve carried out on the lesion site and on the segments immediately proximal and distal to it. The SFI progressively improved for both treated and untreated nerves but in a more marked and significant way for the treated nerves (73 and 55%, respectively). Nerve fiber density did no return to normal in either case but was significantly higher in the treated nerves, with predominance of small diameter thin myelin sheath fibers typical of nerve regeneration in the treated nerves, as opposed to large diameter thin myelin sheath fibers in the untreated nerves. The authors conclude that low intensity therapeutic ultrasound enhances nerve regeneration, as demonstrated with significance on the 21st postoperative day.