Intracellular recordings were made in the CA1, CA3 and dentate cell layers prior to, during and after the bath perfusion of 50 microM carbachol on hippocampal slices. Fifty-six percent of the cells in this sample were termed theta (theta)-related, i.e., they exhibited membrane potential oscillations of 5-28 mV and rhythmic spike discharges related to the carbachol-induced extracellular theta-rhythm. The remaining 44% of the cells did not show the above relationships to the extracellular theta-rhythm. Carbachol produced an overall depolarization in all cells, in the range of 10-20 mV. These results demonstrated the cellular basis of carbachol-induced theta in hippocampal slices. This preparation will be a valuable model for studying cellular mechanisms and network properties underlying electroencephalographic activity.