Synaptic boutons on the somata of rat hypoglossal neurones have been stained with Zinc Iodide-Osmium, and counted using light microscopy. Two weeks after Silastic cuffs containing colchicine or vinblastine were applied to the hypoglossal nerve, the number of boutons on parent nerve cell bodies was significantly lower than controls. These drugs inhibited retrograde axonal transport of horseradish peroxidase, but did not affect neuromuscular conduction or the number of axons in the nerve trunk. Cuffs containing lignocaine did not affect boutons or axonal transport significantly.