Repetitive impulse activity in somatic axons produces a Na-K- ATPase dependent hyperpolarization and a progressive reduction in axonal conduction velocity. Hitherto the hyperpolarization has been deemed causal for the reduction in conduction velocity. This putative causality was examined in single unmyelinated axons innervating the rat cranial meninges in vitro. Action potentials were recorded from the spinosus nerve. Functionally single axons were stimulated electrically at a site close to their terminal with a custom built electrode. The Na-K-ATPase was blocked with Ouabain (10mM–1mM), by cooling and by reducing substrate either by lowering extracellular K+, by replacing extracellular Na+ with Li+ or by reducing the available ATP with cyanide (100–500mM). In 42 axons Na-K-ATPase blockade increased the magnitude of activity-induced conduction velocity slowing thereby demonstrating that Na-K-ATPase dependent hyperpolarisation is not causal for activity-induced conduction velocity slowing. However, the sodium channel blockers lidocaine, carbamazepine and phenytoin (10–500mM) dose- dependently affected activity-induced slowing. At low doses activity-induced slowing was slightly increased while at higher doses activity-induced slowing was decreased and eventually blocked. Activity-induced conduction velocity slowing is therefore proposed to be due to the progressive inactivation of sodium channels.