The effect of hyperbaric oxygen (HBO) treatment on regeneration of the rat sciatic nerve was studied. The sciatic nerve was crushed with a pair of pliers and the animals were either left untreated or subjected to a series of 45-min exposures to 100% O2 at 3.3 atm absolute pressure at 0, 4, and 8 h postoperatively and then every 8 h. Regeneration was evaluated using the pinch-reflex test at 3, 4, or 5 days following surgery and with neurofilament staining at 4 days. The regeneration distances at all time points were significantly longer in animals exposed to hyperbaric oxygen treatment independent of the evaluation procedure. A short initial period of the same HBO treatment schedule, with no more treatments after 25 h, appeared as effective as when treatments were maintained being given every 8 h until evaluation. We conclude that HBO treatment stimulates axonal outgrowth following a nerve crush lesion.