Paucisynaptic connections between n. lateralis posterior (LP) and dorsal hippocampus (Hip) have been demonstrated in acute experiments. In the free moving cats, with chronic implanted electrodes on the neocortex, in LP, pulvinar (Pul), VA and VL nuclei of the thalamus and in the dorsal hippocampus (Hip), the theta waves recorded in LP, Pul and Hip were not of the same frequency and origin. In wakefulness and in slow-wave sleep, the theta waves were of 3.5-4 c/s and occurred exclusively or predominantly in Pul-LP, while in paradoxical sleep they were of 5-6 c/s, occurred' in Hip and invaded the Pul-LP. These two types of theta waves represent two distinct modalities of their organization and reflect the involvement of different mechanisms in the different phases of sleep.