Stable intracellular recordings were obtained from neurones in slices of the guinea-pig olfactory cortex maintained in vitro. 2. Single stimuli applied to the lateral olfactory tract (l.o.t.) produced an excitatory post-synaptic potential (e.p.s.p.) usually generating a single spike. 3. The e.p.s.p. was followed by a long (200-500 msec) after-depolarization (l.a.d.) of peak amplitude 5-16 mV. This was accompanied by a very large conductance increase and was associated with an inhibition of the intracellularly recorded e.p.s.p. and of spike generation. 4. The l.a.d. was more susceptible than the e.p.s.p. to depression by (i) repetitive l.o.t. stimulation and (ii) raising external [Mg2+]. The l.a.d. could be generated without a preceding spike. 5. At an average resting membrane potential of -74 mV the average reversal potential for the l.a.d. (El.a.d.) was -63 mV.El.a.d. became more positive on reducing [Cl-]out or on using KCl-filled electrodes. 6. It is concluded that the l.a.d. represents a Cl- -mediated inhibitory post-synaptic potential, generated through deep-lying recurrent inhibitory loops.
