There is evidence to suggest that the reticular activating system may have an influence on the neurons of the preoptic area (POA). We studied the responses of POA neurons and their relation to the cortical EEG, on stimulation of the midbrain reticular formation (MRF) at various frequencies, in unanesthetized, encéphale isolé cats. Stimulation of the MRF at high frequency produced inhibition of the firing rate in a majority of the responsive neurons of the POA. Stimulation of the MRF with lower frequencies, on the other hand, induced excitation in a majority of the responsive POA neurons. The possibility exists of a summation of stimuli and an involvement of structures bringing about changes in the EEG in causing the shift in the POA neuronal excitability.