A slow outward current has been demonstrated in Aplysia giant neurons which serves to suppress repetitive firing. The barbiturates phenobarbital and pentobarbital enhance the slow outward current and the suppression of repetitive firing. In this study, the effects of diphenylbarbituric acid, which shows anticonvulsant activity in mice and rats but possesses minimal sedative properties, were tested on slow outward current and firing behavior. Diphenylbarbituric acid enhances slow outward current and suppresses repetitive firing at lower concentrations than phenobarbital and pentobarbital. Because diphenylbarbituric acid is effective at enhancing slow outward current but does not produce sedation, this property of barbiturates is apparently not associated with the sedative properties of these drugs, but rather is important for the anticonvulsant effects.