Diphenylhydantoin (DPH) has been shown to prolong the repolarization phase of action potentials in Aplysia and to reduce the spike after hyperpolarization. We voltage-clamped somata of Aplysia giant neurons and measured the action potential currents to determine what alterations might be responsible for the prolonged repolarization and reduced undershoot. DPH did not affect the peak inward current; however, it did reversibly reduce the outward current at concentrations of 38 microM or greater. The data indicate that DPH reduced the voltage sensitive potassium conductance rather than the 'A' current or the calcium activated potassium conductance. These changes in outward current would explain the prolonged action potentials and reduced afterhypolarization seen with DPH in Aplysia.