Diphenylhydantoin effect on excitability in the molluscan giant neuron was investigated. This model of neuronal excitability was convenient for investigating drug mechanisms because the giant somata of these neurons have multiple current systems and can be voltage clamped. Current and voltage clamp methods were used to quantitate firing frequency and the slow membrane currents associated with changes in firing rate. Diphenylhydantoin enhanced the early rapid deceleration of firing of the cell in response to depolarizing current and blocked an early show inward current.