A study has been made of the effect of barbiturates on membrane constants and synaptic potentials of neurones in the isolated guinea-pig olfactory cortex slice. 2. Normally, a long depolarizing i.p.s.p. follows the e.p.s.p. Pentobarbitone (0.1 mM) produced a tenfold increase in the duration of the high conductance phase of this i.p.s.p. 3. The i.p.s.p. was potentiated increasingly with higher barbiturate concentrations from 0.02 to 1.0 mM-pentobarbitone and 0.2 to 5 mM-phenobarbitone. 4. The resting membrane conductance, the initial phase of the e.p.s.p. and the threshold for the action potential were unaffected at lower concentrations. 5. The highest barbiturate doses increased the resting membrane conductance. This was associated with a depolarization of about 14 mV maximally and resulted in smaller synaptic potentials. The effect was probably generated by the same mechanism as the i.p.s.p. 6. This fortifies the idea that barbiturates have a primary action on prolonging inhibition rather than a depression in the excitatory potential.
