The effect of nerve impulses upon the axonal microtubules was studied in the sural nerve of the cat. We showed that a sustained activity (40 impulses/sec for 2 h) determines a difference in microtubule counts: the active nerve exhibited 24% more profiles than the resting one. In addition, microtubules of the active nerve appear to be less sensitive to the depolymerizing action of colchicine. We propose that the action potential leads to a decrease of the disassembly rate constant of microtubules.