Retrograde axoplasmic transport, demonstrated by the "double ligature method" was studied in vitro in the motor neurons of rabbit vagus nerve, labelled with 3H-leucine. When a pressure barrier with a maximal pressure of 30 mmHg, induced by a fluid jet, was applied to a small section of the nerve at 38 degrees C, retrograde flow was partially inhibited. More radioactivity was found in front of the barrier (with respect to direction of flow) than immediately behind it. This observation differs from findings in anterograde transport at the same pressure and tested with a similar technique, where most radioactivity was detected immediately behind the barrier (Hahnenberger 1978).