Motoneurone death induced by axotomy in the rat was studied following section of the medial gastrocnemius nerve near the muscle 4 days after birth. 2. The maximum twitch tension of the medial gastrocnemius muscle achieved by motor reinnervation after section of its nerve was about 70% of that measured on the contralateral, intact side. 3. The number of motor units counted at 35-45 days of age in the animals whose medial gastrocnemius nerves had been sectioned on day 4 was 62% of that observed in normal rats. 4. The number of medial gastrocnemius motoneurones retrogradely labelled with horseradish peroxidase (HRP) 30-40 days after section of the medial gastrocnemius nerve was 77% of that labelled on the contralateral, intact side. 5. When the medial gastrocnemius nerve had been sectioned on day 4 and prevented from peripheral reinnervation, the number of medial gastrocnemius motoneurones labelled with HRP was, on the average, only 18% of that labelled on the control side. 6. Decreased number of medial gastrocnemius motoneurones labelled with HRP following prevention of peripheral reinnervation was associated with a decrease in the neurone density of the motor cell column, indicating the occurrence of motoneurone death. 7. The majority of medial gastrocnemius motoneurones axotomized 4 days after birth appear to maintain their survival for about 2 weeks without target contact. 8. The area of the compound action potential of medial gastrocnemius motor fibres once decreased after axotomy on day 4 began to recover from the 12th day after the operation if reinnervation by the cut peripheral nerve had been allowed, whereas the compound action potential continued to decrease in those axotomized motoneurones whose reinnervation had been prevented. 9. It is concluded that target dependence of motoneurone survival previously observed at embryonic stages is still present during the early post-natal period.
