The influence of the anti-epileptic drug, valproate, on K conductance (gK) was investigated in voltage-clamped Ranvier nodes of Xenopus laevis. A double pulse method was used in order to eliminate the effect of accumulation of potassium ions in the perinodal space, thus enabling the determination of the 'true' magnitude of gK. Valproate (2.4 mM) had a voltage-dependent action on the magnitude of gK. With small step depolarizations more negative than about -50 mV, valproate increased gK (20 ms after the step) to approximately 12% of the maximal gK, an increase which disappeared due to a relatively rapid (less than 200 ms) inactivation process. However, with step depolarizations more positive than about -50 mV, valproate markedly reduced gK (20 ms after the step) at greater depolarizations, with a maximum of about 40% of the maximal gK. Moreover, at these voltages gK was inactivated completely (less than or equal to 10 s), whereas under control conditions the inactivation was only partial. Both the temporary increase and the steady state decrease of gK could contribute to an anti-epileptic effect by increasing the action potential threshold and by preventing excessive depolarizations of the nerve during epileptic seizures, respectively.