Three to four months after irradiation, the ratio of axon area and total area of the cross-sectioned nerve was shifted towards the axon area. Seven to 9 months after completion of irradiation, both the myelin sheath and the axon were reduced in diameter. The loss of substance affected predominantly the myelin sheath. Peripheral nerves are radiosensitive. Seven to 9 months after irradiation there was an increase of the axon area compared to the total area, both in absolute values and in relative terms. Radiosensitivity of peripheral nerves can be measured morphometrically in suitable animal models. The measurements disclosed temporal and spatial patterns of radiation response of the nerves. These results may partly explain the increased radiosensitivity of the peripheral nervous system observed in the long-term follow-up after completion of radiotherapy of head and neck in humans.