Infection has been obtained in both rhesus and cynomolgus monkeys by inhalation of poliomyelitis virus in the form of droplet nuclei. The olfactory route was excluded in part of the animals successfully infected. The gastrointestinal route is believed to have been excluded in the rhesus monkeys. It seems most probable that the portals of entry were the lower respiratory mucosa in the case of the rhesus monkeys and the oropharyngeal mucosa in the case of the cynomolgus monkeys. Fever and occasional mild symptoms in 8 other rhesus monkeys suggest that an abortive form of poliomyelitis may have resulted from inhalation, but this can not at present be considered as proved. The experiments open up the possibility that human poliomyelitis may, at least sometimes, be an air-borne infection and that the lungs may be a portal of entry. Neither of these aspects of the disease has hitherto, so far as we are aware, been studied experimentally. The presence of virus in the human nasopharynx, which has been repeatedly demonstrated,(7, 8) provides an obvious source of air contamination by patients and carriers; and direct contact has been traced during epidemics in a considerable fraction of cases,(9) amounting to about one third in the report of Top and Vaughan.(10) The relative importance of transmission by air and by ingested material remains to be determined. It would seem probable, however, that both modes of infection must be taken into account.