The relative importance of ischaemic and direct mechanical injury to nerves compressed by a tourniquet, in the pathogenesis of tourniquet paralysis in man has not been established. To investigate this question, conduction in ulnar or median nerve fibres has been measured in healthy subjects both at the level of the pneumatic tourniquet and distal to the tourniquet. Measurement was prior to, for the period of tourniquet inflation, and following release of the tourniquet. The earliest conduction delays and block were observed at the level of the tourniquet, particularly across the proximal tourniquet border zone. However, a proximal to distal progression in conduction abnormalities distal to the tourniquet suggested that the earlier conduction abnormalities at the level of the tourniquet were primarily ischaemic in origin. Mechanical compression, however, probably contributed to disproportionate conduction delays and blocks across the border zones of the tourniquet.