The effects of phasic auditory and visual stimuli upon single unit activity of serotonergic neurons within the nucleus raphe dorsalis (RD) and nucleus raphe pallidus (RPA) were studied in freely moving cats. RD units were driven by both auditory and visual stimuli, while RPA neurons were totally unresponsive to auditory and visual stimulation. These data support the hypothesis that ascending and descending serotonergic neurons subserve different functions.