Deltamethrin produced a prolonged period of increased excitability following the passage of a nerve impulse, which was dose-related and lasted up to 400 ms. Excitability changes were detected without neurological signs following a single IV injection of 0.5 mg/kg and feeding 200,100 and 50 ppm in the diet for up to 8 weeks. No changes were detected following 0.3 mg/kg IV or 25 ppm in the diet. No cumulative effects were detected during chronic feeding. Cismethrin produced increased nerve excitability only between 2 and 4 ms after a nerve impulse. Excitability changes after cismethrin were biphasic during the first 20 ms.