Newborn rat respiratory activity was recorded on hypoglossal nerve and ventral cervical roots during in vitro experiments performed on superfused brainstem spinal cord preparations. The addition of serotonin (5-HT) to the bathing medium increased the respiratory frequency and selectively depressed the hypoglossal activity. Any decreases in the amplitude of cervical recordings were always restricted and reversible, whereas the hypoglossal activity was abolished. Furthermore, on cervical roots, 5-HT induced a tonic activity superimposed on the respiratory one, which was never observed with the hypoglossal nerve. When 5-HT was applied on isolated hemispinal cord, a tonic activity could still be elicited. These results indicate that serotonin (i) modulates the activity of neurons involved in the generation of respiratory rhythm, (ii) depresses the activity of hypoglossal motoneurons, and (iii) evokes tonic activity in cervical motoneurons, probably as the result of direct spinal effects.