The topography of spontaneous and hypothalamically induced hippocampal rhythmical slow wave activity (theta) was studied acutely in cats anesthetized with urethane. Tracking and depth profile analysis using microelectrodes showed two amplitude maxima of theta activity approximately 180 degrees out of phase separated by a null zone. One amplitude peak was located in stratum oriens of CA1 (maximum amplitude 1.2 mV) and the other peak in stratum moleculare of the fascia dentata (maximum amplitude 1.9 mV). The null zone was localized to stratum radiatum, just ventral to the CA1 pyramidal cells. The two 'generator' hypothesis of theta activity was discussed in relation to similar findings for other species. Pharmacological results were interpreted as supporting the view that there is an ascending cholinergic input mediating theta in the urethanized cat.