In a series of 16 patients on long-term lithium therapy pathological changes were found in peripheral nerve conduction. Such changes correlated with the duration of lithium therapy (mean: 8.3 years, range: 3-15 years; p less than 0.05). No patient showed clinical signs of polyneuropathy or any other disease known to be frequently associated with peripheral nerve lesions. Compared with an age-matched normal control group (n = 27) the nerve conduction velocity of the sural nerve was reduced (p less than 0.05), for which a chronic neurotoxic effect is considered to be the most probable mechanism.