In thirty-five acute cat preparations under Flaxedil and in five chronic cat preparations, two neural systems corresponding, respectively, to hippocampal theta and to hippocampal desynchronization, have been identified and delineated with respect to origins in the mesencephalic reticular formation and their separate pathways via the medial or lateral hypothalamus to the hippocampus. The origins of these two contrasting systems were found to be adjacent in the dorso-lateral mesencephalic tegmentum, with the theta system localized in a slightly more dorso-lateral region. The medial hypothalamic pathway, believed to involve the dorsal fasciculus of Schütz, when stimulated at 100/sec, produced hippocampal theta activity; when this pathway was blocked by electrolytic lesions or reversible functional cryogenic blockade in more anterior regions of the hypothalamus, hippocampal theta waves were eliminated. In contrast, stimulation of the lateral hypothalamic pathway (believed to involve the medial forebrain bundle) caused hippocampal desynchronization which was blocked by electrolytic lesions or cryogenic blockade in more anterior regions. The theta system was blocked by lesions of the dorsal fornix, but the desynchronization system was unaffected, thus indicating further the distinctiveness of these mechanisms and of their separate pathways. Confirmation of the electrophysiological results was obtained in chronically awake and behaving preparations. Induction of theta activity in the hippocampus by stimulation of the medial hypothalamic theta system produced orienting and searching behavior. Blockade of the lateral hypothalamic desynchronizing system had a similar effect upon behavior. In contrast, stimulation of the lateral hypothalamic system or cryogenic blocking of the medial hypothalamic system produced an arrest of ongoing behavior and caused an attentive fixation of gaze. These results are deemed encouraging in the further analysis of natural or specifically trained behaviors as they relate to the distinctive hippocampal theta and desynchronization patterns.