Intracellular recordings were made in the dorsal hippocampal formation of urethane-anesthetized rats as the local field activity spontaneously cycled between a synchronous condition termed theta and an asynchronous condition termed LIA. All cells reported in this study were labeled with Neurobiotin and classified as theta-related or non-theta-related according to the system of Colom and Bland [Colom, L.V., Bland, B.H., 1987. State-dependent spike train dynamics of hippocampal formation neurons: evidence for theta-ON and theta-OFF cells. Brain Res. 422; 277-286]. The findings are the first demonstration that hippocampal pyramidal cells are functionally heterogeneous in relation to the generation of theta-band oscillation and synchrony. In field CA1 pyramidal cells formed theta-related subsets of phasic theta-ON cells and tonic theta-ON cells and non-theta-related subsets of simple spike discharging cells, complex spike discharging cells and "silent" cells. Similar findings were evident for CA3 pyramidal cells.