Abstract The in vitro brain slice technique has been applied to the study of the neocortex. Cortical blocks were removed from adult rats deeply anesthesized with halothane, sectioned coronally at 400–700 μm, and placed in a brain slice chamber. Cortical slices typically showed spontaneous and evoked potential activity and normal histology for 8 h or longer. Single units and evoked potential recordings were made from different layers of the cortex using micropipettes. The evoked potentials to electrical stimulation of differing intensity, frequency, and from different cortical layers were analyzed. Evoked potential from all but the most superficial layers of the cortex showed a characteristic 6-component response to stimulation of nearby white matter. This evoked potential closely resembled cortical responses recorded in vivo by other investigators following afferent stimulation. The response amplitude of all components increased as stimulus intensity was raised. Radial movement of the recording electrode showed that components 1–3 had their largest amplitudes in the deepest cortical layers, component 4 reached its greatest amplitude and shortest latency in layer IV, and components 5 and 6 reached their greatest amplitudes in layers IV to II. The frequency following for various components was measured showing greater decline in amplitude for components 4–6 than 1–3. This, together with the results of previous investigators, suggests that the first 3 components represent afferent fiber input, while component 4 represents the first cortical response (layer IV). Components 5 and 6 represent later, additional cortical responses. Further support for the intracortical origin of component 4 was provided by lateral intracortical stimulation within layer IV, giving an evoked potential composed mostly of component 4. With lateral movement of the recording electrode in layer IV the evoked potential disappeared in under 1 mm, suggesting a fairly restricted afferent input to the cortex. The present results encourage the use of the cortical brain slice preparation as an appropriate model system in which to study cortical neural circuitry.