1. Five different methods have been used to estimate the rate of advance of regeneration along nerves in the rabbit. Not all give the same result. When the advance of the axon tips is studied a higher rate is recorded than when the times necessary for functional recovery are considered. In the latter case the experiment gives a measure of the rate of advance not of the tips of the axons but of completed nerve fibres, able to function.

2. The ‘scar delay’, or period before arrival of new axon tips in the peripheral stump, is nearly constant in all well-made sutures (7.3 days). After interruption of a nerve by complete crushing at one spot this delay is only slightly shorter (5.2 days).

3. There is a further delay before maturation to the level of functional completion begins to advance down the nerve, so that the total latent period before advance of recovery begins is about 36 days after suture and 20 days after crushing a nerve.

4. The rate of advance of the fastest axon tips was determined by finding the furthest point from the lesion at which reflexes could be elicited by pinching the nerve. After suture of the n. peroneus in the rabbit this rate was 3.5 and after crushing 4.4 mm./day.

5. This rate of the axon tips is not significantly different after crushing the nerve at different distances from the nerve cell body, namely high in the thigh or below the knee.

6. This rate is similar in the tibial, peroneal and sural divisions of the sciatic in the rabbit.

7. The rate of advance of functional, completion of nerve fibres was measured by making lesions at various levels and studying (a) the time necessary for return of response to nociceptive stimuli to a given point on the skin and (b) the time for recovery of a given motor function, namely, spreading of the toes. Both methods give rates of 2.0 mm./day after suture, and 3.0 mm./day after crushing.

8. Analgesic areas on the foot shrink at the rate of 2.1 mm./day during recovery after crushing the nerve, 1.6 mm./day after suture.

9. Comparison of the times of recovery of muscles at varying distances from a lesion gives an estimate of rate of advance of functional completion of 2.2 mm./day after crushing a nerve, but this method is only approximate in the rabbit.

10. In rabbits of 1 month old the rate of advance of the axon tips is not greatly different from that in adults, but the delay in the scar is less, and maturation of the nerve fibres proceeds more rapidly.

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