The effects of X-irradiation on the regeneration of peripheral nerve across a gap were studied in the mouse. The right sciatic nerve was transected and 3 days later a 20-Gy dose of X-rays was administered to the hind limb, which was shielded so that the irradiation affected either the proximal, the distal or both parts of the nerve. Within a few hours of irradiation, the proximal and distal nerve stumps were inserted into a polythene tube within which they were separated by a 5-mm gap. After 1 or 2 months the sciatic nerves were processed for examination by light and electron microscopy. Irradiation affected axonal growth, differentiation of Schwann cells and formation of a perineurium. These effects were most marked when both the proximal and distal stumps had been exposed to X-rays. At 2 months there were fewer axons in the irradiated than in control nerves (P less than 0.01), but all of the larger axons which had regenerated were myelinated, albeit more thinly than in the controls. The changes in the pattern of regeneration in the irradiated nerves are interpreted as being largely due to radiation damage to Schwann cells.