SummaryThis study is concerned with the effect of colchicine on the structure of fibrillar constituents of neurons and on the transport of neuronal organelles. Colchicine was injected beneath the perineurium of the hypoglossal nerve. Close to the site of the injection, the 24 and 48 h experimental axons showed loss of microtubules, increased numbers of filaments and increased amounts of the microfilamentous material bridging the filaments. Evidence of organelle damming was found proximal to the site of the injection. Five days after the injection of colchicine the nerves appear to have recovered and resemble control nerves. It is speculated that the circumscribed increase in the amount of filaments and microfilaments may produce a 'gel barrier' that interrupts mechanically the movement of organelles.