Neuronal activity of the medial septum (MS--DB) and hippocampal slow electroencephalogram (EEG) were recorded in unanaesthetized rabbits with stimulating electrodes implanted in the median raphe nucleus and reticular formation of midbrain and pons. In all the animals with raphe electrodes the background frequency of theta-rhythm was decreased (mean 4.7 +/- 0.15 Hz) in comparison with the intact animals and those with stimulating electrodes in the reticular formation only (5.2 +/- 0.19 Hz, p < < 0.5). Stimulation of the raphe had effects of suppression of theta-bursts, decrease in their frequency and regularity, and appearance or increase of delta-modulation in MS--DB neuronal activity. The number of spikes per burst was decreased while interburst pauses became significantly longer. At the same time, gradual decrease of theta frequency and amplitude, theta suppression, appearance of irregular delta-waves and spindles were observed in the hippocampal EEG. Stimulation of the reticular formation evoked the opposite effects in MS--DB neuronal activity, as well as in the hippocampal EEG. The data confirm the role of the midbrain raphe in constraining the reticulo-septohippocampal system of theta-rhythm generation and do not support the statements on the existence of septum-independent high-frequency serotonergic theta-rhythm. It is suggested that theta-limiting raphe influences may be important for suppression and switching of attention.