The effects of sodium pentobarbital were studied using intracellular recordings from CA₁ and CA₃ pyramidal cells in slices of guinea pig hippocampus. Drugs were applied either by perfusion or by pressure ejection at concentrations of 10⁻⁶, 10⁻⁵, and 10⁻⁴ M. Pentobarbital at all concentrations caused neuronal hyperpolarization, decreased spontaneous activity, and sometimes decreased input resistance. Hyperpolarization also occurred in zero calcium perfusate or with tetrodotoxin in the perfusate. The postspike train long-lasting afterhyperpolarization, which is an intrinsic calcium-mediated potassium conductance, was increased at all doses. γ-Aminobutyric acid induced depolarizing dendritic responses were augmented only at 10⁻⁴ M pentobarbital. It is proposed that one of the important mechanisms of pentobarbital neuronal inhibition, particularly at lower doses, is an increase in potassium conductance.