By using a radially directed stream of fluid it is possible to compress a section of a nerve in vitro by known pressures without interrupting the supply of oxygen and nutrients to the nerve. Motor neurons of the vagus nerves of rabbits labelled with 3H-leucine were used at room temperature. When a nerve was subjected to 20 mmHg pressure fast axoplasmic flow was not altered, but at 30 mmHg there was a slight but consistent inhibition, which was even more marked at 60 mmHg and still more at 90 mmHg. The pressure induced block of axoplasmic flow was reversible when 60 mmHg pressure was applied for as long as 4 h, when the nerves were compressed for 15 h there was reversibility in only 5 out of 8 cases.