Rhythmic slow wave activity recorded in the ventral mesencephalic tegmentum in the rat.

1. Slow rhythmic theata-type waves were recorded in the sub-interpeduncular ventral mesencephalic tegmentum (VMT) in the rat. This region is one of the parts of the "limbic midbrain area". These waves are recorded during certain attentive waking states, during the execution of voluntary movements and during paradoxical sleep. 2. These rhythmic VMT waves seem to be of the same nature as the theta rhythm recorded in the limbic structures. A lesion of the septum suppresses the slow rhythmic waves at the mesencephalic level as well as at the telencephalic level. 3. Although we have now shown that the mesencephalic and telencephalic slow rhythmic waves have the same pacemaker, the pathways which transmit the theta rhythm to the VMT are not well known. However, it is reasonable to suppose that the fibres of the post-commissural fornix are involved in this transmission.