The aim of this work was to study the nonvagal mechanisms which might induce apneic reflexes in kittens. Experiments were performed on spontaneously breathing animals (19 kittens at different postnatal ages). Animals were anesthetized (halothane) or decerebrated by transcollicular section of the brainstem. Weakly nociceptive cutaneous stimulations and various oral stimulations were administered in all animals. In 10 kittens, one of the lingual nerves was stimulated electrically. In 3 decerebrate kittens the effects of serotonin on respiratory activity and on muscles innervated by the hypoglossal nerve were studied. All the stimulations produced apneas of variable duration, and expiratory reinforcement was associated with activation of pretracheal muscles. Similar effects were observed after applying serotonin to the floor of the 4th ventricle. Thus serotonin may be involved in the mechanisms that cause some apneas.