1. Silices of guinea-pig olfactory cortex were cut at 550 micrometer nominal thickness and preincubated at 24 +/- 0.5 degrees C for than 2 1/2 hr. They were then stimulated via the lateral olfactory tract, and field potential recordings were made from all regions of the slice. 2. Potentials recorded resembled those described previously, but it was noticed the early N-wave had two distinct components, which we designated the N'a' wave (earlier) and N'b' wave (later). Evidence was obtained that this was not a consequence of the division of a single population e.p.s.p. (N-wave) into two by a P notch (synchronous discharge of post-synaptic action potentials). 3. In some slices the N'a' wave and N'b' wave had similar thresholds, and in others the N'a' wave had the slightly lower threshold. 4. The N'b' wave was best developed at low frequencies of stimulation (less than 0.1 Hz), and considerably depressed with stimulation above 1 Hz. This was most evident with submaximal stimulation. 5. Exploration of the distribution of peak amplitudes and latencies of the N'a' and N'b' waves showed that the N'a' wave could have been directly initiated by lateral olfactory tract action potentials, while the N'B' wave could not. The N'b' wave amplitude was relatively larger towards the periphery of the slices, away from the tract. In a few cases, an N'b' wave could be recorded in the absence of an N'a' wave at that site. 6. Depth studies showed that the origin of the N'b' wave lay deeper in the slice than that of the N'a' wave. 7. The effect of conditioning stimulation on the N'a' and N'b' waves was examined. The N'b' wave was more depressed at short conditioning intervals than the N'a' wave, and showed less later potentiation. The recovery of the N'b' wave from conditioning was much slowed with submaximal stimulation, and when trials were repeated at low frequency. 8. The N'a' and N'b' components persisted when the slice was warmed to near-physiological temperatures, and showed a similar pattern of response to conditioning stimulation as had been found at lower temperatures. 9. N'a' and N'b' waves could still be recorded when slices were incubated in a medium containing 1.2 mM-Mg2+ and 1.2 mM-Ca2+. These physiological concentrations were about half those routinely employed. There was little or no depression of the N'b' component by conditioning stimulation in this medium. 10. The N'a' wave is probably a result of e.p.s.p.s in apical dendrites of superficial pyramidal cells, initiated by transmitter release from lateral olfactory tract axon collaterals. The N'b' wave may reflect e.p.s.p.s in the apical dendrites of deeper pyramidal cell elicited by firing in recurrent collaterals from superficial pyramidal cell axons.