A quantitative study of Wallerian degeneration was carried out on teased fibres. The breakdown into ovoids was used as the criterion of degeneration. The fragmentation of fibres begins near the point of nerve interrruption and spreads along the unbranched parts of axons at velocities correlated with fibre diameter and internodal length. The latent period, preceding the onset of fragmentation, lasts from 25.6 h in thin fibres to 45.0 h in the thickest fibres. The speed of the subsequent advance along the nerve varies correspondingly from 250 to 46 mm/day. In each internode the first ovoids appear in the middle, the ends of the internodal segment being initially spared. The spatiotemporal pattern of degeneration is consistent with the hypothesis that a neuronal trophic substance, normally ensuring the integrity of the axon, exerts transcellularly an inhibitory influence on the Schwann cell. This influence disappears when the amount or concentration of migrating trophic substances falls below a critical level in a stretch of axon. The overlying Schwann cells become mobile and exhibit intense metabolic activity, leading eventually to axonal disruption.