1. Action potential conduction along frog sciatic nerve fibres has been monitored during compression of a mid-portion of the nerve. 2. The effects of compressing a 24 mm length of nerve with a pressure of 250 mmHg applied pneumatically were investigated by recording unitary action potentials. A plot of time before conduction failure (survival time) against initial conduction velocity revealed that the faster myelinated axons tend to fail before the slower myelinated axons. A large degree of scatter was evident in the pooled data as well as in the data from individual experiments. 3. When the compression was made more severe by increasing the applied pressure to 750 mmHg, the order of block was reversed, i.e. the slower myelinated axons tended to block first. Similar scatter in the order of conduction block was observed. 4. The average survival time of units following application of compression was considerably different between these two series of experiments. When 750 mmHg pressure was applied, units survived for, on average, 10.9 min (n = 246). When 250 mmHg pressure was applied units survived for, on average, 50.4 min (n = 148). 5. The results are discussed in relation to the underlying causes of conduction failure as a result of compression and in relation to results from previous investigations.