In these experiments the firing patterns of neurons located throughout the CA1-dentate region were recorded during the occurrence of either larger amplitude irregular activity (LIA) or slow wave theta activity, in the hippocampal formation of urethane-anesthetized rats. The main conclusion was that there were two distinct populations of neurons whose spike train dynamics varied systematically in relation to the ongoing EEG state and therefore probably played a role in the genesis of these slow wave rhythms. Within each of the two populations, called theta-on and theta-off cells, there were several subtypes labelled tonic and phasic. Tonic subtypes provided information about the change from one EEG state to another and phasic subtypes, in addition to showing this, provided specific information about frequency changes within the theta state. The importance of extending these observations to freely moving animals was discussed.