Abstract The membrane potential of olfactory cortex and neocortex neurones in vitro was recorded using conventional microelectrode techniques. During recordings with KCl- or CsCl-filled microelectrodes, spontaneous, subthreshold, transient membrane depolarizations were observed. These were abolished by the GABA A-receptor antagonist, bicuculline methiodide, and were prolonged by the barbiturate pentobarbitone. In most cells they were abolished by tetrodotoxin. It is concluded that these spontaneous depolarizations are inhibitory postsynaptic potentials arising from spontaneous activity in inhibitory interneurones.