Intracellular recordings were made from the fast bender excitor motor axon in autotomized crab limbs bathed in normal saline, and in salines made with up to 240 mM of ethanol. The presence of ethanol reduced the amplitude, the rise time and the decay time of the evoked action potential, and decreased the velocity at which the spike was conducted down the axon. There was a linear relationship between each of these four parameters and the concentration of ethanol in the saline. The close relationship between spike rise time and conduction velocity suggests that ethanol slows the rate of membrane depolarization by the spike and thus decreases the velocity at which action potentials are propagated along the axon.