AbstractRabbit vagus nerves were treatedin vitrowith colchicine and halothane, an anesthetic, and the electrical activity monitored. The number of microtubules per square micron of axonal area was measured for each treatment directly from electron micrographs with the aid of a planimeter. Colchicine (10 mM) significantly reduced the number of microtubules per square micron of axonal area from 40 to 16 and had little effect on the electrical activity. Halothane (3 and 10 mM) significantly increased the number of microtubules from 40 to about 55 per square micron of axonal area and blocked the ability of the nerve to propagate impulses within 25 and 10 min, respectively.