Dorsal and ventral nerve roots from clinically normal dogs aged 3 months to 13 years have been examined. The L6, L7 roots were selected and samples removed from the roots within the sub-arachnoid space and within the root sleeves. Routine light microscopy, electron microscopy and single fibre studies have been made. The single teased fibre results have been analysed statistically for the correlation of internodal length and diameter. With increasing age there was marked evidence of demyelination and remyelination. Paranodal and segmental demyelination were present and there were numerous fibres showing a marked variation in internodal length. In younger animals these features were not seen. Wallerian type degeneration was uncommon. In older animals myelin balloons, similar to those described proximal to neuromas, were commonly found. These were present in both dorsal and ventral roots. The balooning appeared to result from a splitting of the myelin sheath. The intervening space was non-staining but sometimes contained myelin debris. No evidence of neuromas was present in these roots or peripheral nerves. These studies suggest that both the ballooning and demyelination and remyelination are age related.