1. Slices of guinea-pig olfactory cortex were maintained in vitro. Electrical properties of neurones in the prepyriform region were studied using single high resistance glass micro-electrodes filled with potassium acetate, connected to a resistance-compensating circuit to allow passage of current through the electrode. 2. Neurones showed a high, stable resting membrane potential (75.4 +/- 2.7 mV, mean +/- S.D.; n = 47). Input resistance measured with small depolarizing currents varied over a range of 9-280 Momega. The time constant for decay of depolarizing potentials was 19.4 +/- 7.5 msec (mean +/- S.D.).. 4. Depolarization produced repetitive action potentials (maximum frequency of 85 Hz) having peak amplitudes of +16 to +47 mV. The action potential was followed by a depolarizing after potential of about 20 mV positive to the membrane potential. 5. In these and other respects, the prepyriform neurones appear to behave like most other neurones in the mammalian brain, after allowing for the more stable recording conditions in this preparation.