The effect of nucleotide administration on the regeneration of myelinated nerve fibres following crush injury to the sciatic nerve of the rat was studied using morphometric techniques. In addition morphometrical investigations of peroneal and soleal muscles were performed at different times. After a localized crush lesion of the right sciatic nerve, rats were given nucleotides daily at a dosage of 3.0 mg/kg body wt uridine monophosphate (UM), 2.5 mg/kg body wt cytidine monophosphate (CMP) or 3.0 plus 2.5 mg/kg body wt UMP plus CMP, respectively. Observations were made after 20, 40 and 60 days of common peroneal nerve regeneration for comparison with age-matched crushed or nonoperated controls. Forty days after daily UMP/CMP administration the single fibre conduction velocity of all type II afferents was significantly accelerated. There was a trend towards increased mean fibre area related to increased myelin area. Mean diameter of type II muscle fibres was increased. After 60 days, there was a trend to increase of single afferent fibre conduction velocity in the UMP/CMP group. In the same group automated morphometry revealed a significant increase of nerve fibre area, myelin area and axon area. At this time an increase was found of type I and/or type II muscle fibres in all animal groups. The present results suggest that both axons (neurons) and myelin sheaths (Schwann cells) of regenerating nerve fibres and regenerating muscle fibres are influenced by nucleotide administration.