The Hoffmann monosynaptic reflex was recorded in the soleus muscle in 105 healthy children aged between 2 days and 5 years. From this the nerve conduction velocity in the proximal segment of the sciatic nerve was calculated. The tibial nerve conduction velocity (motor) was also measured in 59 of these children. The conduction velocity in the proximal and distal parts of the nerve develop in parallel and in an exponential manner but the conduction velocity in the reflex arc (afferent and efferent) is always faster than the motor conduction. Conduction time slightly but significantly decreases during the first year despite the increase in length of the nerve. Thereafter the conduction time slowly increases and by 5 years the values are similar to those in the neonatal period.