Fast anterograde axoplasmic transport was studied in the motor fibres of the vagus nerve of rabbits in vitro at 22 degrees C with a pressure barrier applied to a small section of the nerve. The barrier was characterized by the maximal pressure and the gradients on each side of the maximum. The range tested was 16 to 45 mmHg maximal pressure and 30 to 140 mmHg/mm length of nerve for the pressure gradients. It was found that in the ranges tested the maximal pressure was of major importance for the inhibition of transport, the pressure gradient being of little significance. The results indicate that at least one part of fast anterograde axoplasmic transport occurs within collapsible canalicular structures.