In the present study 99 cells were recorded in the pontine region of urethane-anesthetized rats during: (1) the spontaneous occurrence of hippocampal formation (HPC) theta field activity; (2) sensoryinduced (tail pinch) theta field activity; and (3) large amplitude irregular field activity (LIA). Using the criteria of Colom and Bland (Brain Res 1997;422:277-286) for the classification of theta-related cells, 58/99 cells (59%) were involved with changes in activity related to the occurrence of HPC theta field activity, 24/99 (24%) were non-related, and 17/99 (17%) were related to the sensory input (tail pinch). All cells recorded discharged in a tonic, non-rhythmic pattern in relation to the HPC field activity occurring during the three conditions. Of the 58 theta-related cells, 52 (90%) were classified as tonic theta-ON cells and 6 (10%) as tonic theta-OFF cells. There were no clear regional differences in the distribution of cell types. Statistical analysis of the discharge rates of tonic theta-ON cells during spontaneously occurring theta and tail pinchinduced theta (tested on 48 cells) revealed that 22/48 (46 %) of these cells discharged at significantly higher rates during the faster theta field frequencies associated with tail pinches while 26/48 (54%) tonic theta-ON cells did not change discharge rate between the spontaneously occurring theta and the tail pinch-induced theta states. In addition, the discharges of 11/52 (21%) tonic theta-ON cells exhibited weak to moderate correlations with the negative peak of HPC theta field activity recorded from the stratum moleculare of the dentate gyrus. Of the 17 cells related to the sensory stimulation (tail pinch), 12 (71%) cells increased discharge rate during the tail pinch and were classified as sensory activated, while 5 (29%) cells decreased discharge rate during the tail pinch and were classified as sensory inactivated. The results supported the following conclusions: (1) the main cells in the pontine region involved with changes in activity related to the occurrence of HPC theta field acitivity are tonic theta-ON cells and tonic theta-OFF cells; (2) a subpopulation of tonic theta-ON cells coded the increasing intensity of activation of the ascending brainstem HPC synchronizing pathways by an increase in discharge rate; and (3) a smaller population of cells in the rostral pontine region appeared to be related to sensory stimulation, independent of thetarelated activity.