The anesthetic-sedative barbiturate pentobarbital (PB) and the anticonvulsant barbiturate phenobarbital (PhB) were applied to mammalian spinal cord neurons in primary dissociated cell culture to assess their effects on: (1) postsynaptic GABA-responses; (2) paroxysmal activity produced by the convulsant bicuculline; (3) resting membrane properties; and (4) spontaneous neuronal activity. The results demonstrated that: (1) anticonvulsant actions occurred at barbiturate concentrations which augmented GABA-responses; (2) anesthetic actions occurred at barbiturate concentrations which produced direct increases in chloride conductance; (3) both anticonvulsant and anesthetic actions occurred at clinically relevant concentrations; and (4) concentrations of PhB, but not PB, which produced GABA-augmentation and direct conductance changes were widely separated. These findings support the hypotheses that augmentation of GABA-mediated inhibition and possibly reduction of glutamate (GLU)-mediated excitation form the basis at least in part for barbiturate anticonvulsant action and that addition of direct increases in chloride conductance to augmentation of GABA-mediated inhibition and reduction of GLU-mediated excitation may partially underlie anesthetic-sedative barbiturate action.