Tension introduced into peripheral nerves during their surgical repair may reduce the success of this procedure. Two mechanical factors are important; the tension required to effect a repair, and the rate at which this tension changes after surgery. These two factors have been investigated in the rat sciatic nerve. The results show an increasing resistance to elongation of the nerves with increasing tension. Under a constant elongation the tension in the nerves reduces by about 30 per cent in the first 10 minutes and by a small amount in the following 20 minutes.