Mechanisms which have been proposed to explain the lung edema following bilateral midcervical vagotomy in guinea pigs include the loss of parasympathetic impulses to the heart and lungs, interference with motor control of laryngeal musculature, loss of sympathetic impulses to the superior cervical ganglia, and interruption of afferent vagal impulses to the brain stem. By means of several surgical manipulations, each of these fiber components of the midcervical vagus nerve was sectioned preferentially at the level of the nodose ganglion. Guinea pigs subjected to one of these operative procedures were compared with animals undergoing midcervical vagotomy or sham operation for differences in lung weights and survival times. The results indicate that bilateral interruption of afferent impulses of the tenth cranial nerve is the factor that initiates vagotomy-induced lung edema.