Adding of 5% bovine serum to internally perfused voltage-clamped serum deprived neuroblastoma cells rapidly stimulates transient sodium current. This stimulating effect is mainly due to the increase in the peak sodium conductance by almost 24 per cent, on the average. Besides that a modifying effect was observed resulting in the 6 mV shift of the sodium peak conductance curve towards more negative potentials and in the 5 mV shift of steady inactivation curve towards more positive ones. The sign of the latter shift was changed to the opposite under the action of serum thermally pretreated at 100 degrees C. This procedure led also to more than two fold lowering of the stimulating effect. Experiments with serum deprivation demonstrate different degrees of reversibility of the serum effects, the most reversible being the inactivation curve shift. EGF, insulin, dexamethasone, transferrin, ATP, serotonin and their combinations in physiological concentrations failed to give the typical whole serum effects. The serum is supposed to contain at least two active components of unknown nature, one of which being thermoresistant.