The effect of sea anemone toxins from Parasicyonis actinostoloides and Anemonia sulcata on the Na conductance in crayfish giant axons was studied under voltage-clamp conditions. The toxin slowed the Na inactivation process without changing the kinetics of Na activation or K activation in an early stage of the toxin effect. An analysis of the Na current profile during the toxin treatment suggested an all-or-none modification of individual Na channels. Toxin-modified Na channels were partially inactivated with a slower time course than that of the normal inactivation. This slow inactivation in steady state decreased in its extent as the membrane was depolarized to above -45 mV, so that practically no inactivation occurred at the membrane potentials as high as +50 mV. In addition to inhibition of the normal Na inactivation, prolonged toxin treatment induced an anomalous closing in a certain population of Na channels, indicated by very slow components of the Na tail current. The observed kinetic natures of toxin-modified Na channels were interpreted based on a simple scheme which comprised interconversions between functional states of Na channels. The voltage dependence of Parasicyonis toxin action, in which depolarization caused a suppression in development of the toxin effect, was also investigated.