1. The effects of the barbiturate anaesthetic pentobarbitone on the membrane properties and amino acid pharmacology of mammalian C.N.S. neurones grown in tissue culture were studied using intracellular recording coupled with bath application, extracellular ionophoresis, or focal diffusion. 2. The addition of an anaesthetic concentration of pentobarbitone to the bathing medium abolished all spontaneous synaptic activity, but did not render individual cells electrically inexcitable nor prevent evoked synaptic acitivity. 3. Focal ionophoresis of pentobarbitone or diffusion from blunt micropipettes reversibly increased membrane conductance, effectively dampening excitability without directly affecting individual action potential characteristics. 4. Pentobarbitone-induced membrane conductance was reversibly blocked by picrotoxin. The inversion potential of the pentobarbitone voltage response depended on Cl- ion gradients and was similar to that of GABA. 5. Pentobarbitone reversibly enhanced the conductance increase produced by GABA with a variable slowing of response kinetics, shifting GABA dose-response curves to the left. Responses to glycine and beta-alanine were not affected. 6. Higher ionophoretic currents of pentobarbitone, which measurably increased membrane conductance, attenuated and markedly slowed GABA responses. Similar effects on GABA responses were observed by superimposing GABA pulses on low level GABA currents. 7. Pentobarbitone, in the absence of an increase in membrane conductance, reversibly depressed depolarizing responses to glutamate without changing response kinetics. Slower responses to acetylcholine which were associated with an apparent decrease in membrane conductance were not affected by the drug. 8. Analysis of double-reciprocal plot data suggested a non-competitive type of antagonism between pentobarbitone and glutamate. Pentobarbitone depression of glutamate was not affected by picrotoxin. 9. Both GABA and glutamate responses appeared to be equally sensitive to pentobarbitone. Specific interaction of the drug with amino acid receptor-coupled events is indicated by the requirement for pentobarbitone pipette placement close to the amino acid response site. 10. The results suggest that pentobarbitone depresses neuronal excitability by (1) directly activating post-synaptic GABA-receptor coupled Cl- conductance, (2) potentiating post-synaptic GABA-induced conductance events, probably at the level of the GABA receptor, and (3) depressing post-synaptic glutamate-induced excitation, probably at the level of the conductance mechanism.