We studied the influence of unilateral vagal stimulation on intrapulmonary neuroepithelial bodies (NEB) in rabbits. The left vagus nerve was cut and electrically stimulated for 10 min. Animals were killed and the lungs studied with fluorescence and electron microscopy. Intensity of formaldehyde-induced fluorescence, which reflects the serotonin content in NEB, was higher on the stimulated side than on the nonstimulated side (118 +/- 7 vs. 100%, n = 8, P less than 0.001). The latter difference was found to correlate with the stimulus amplitude (r = 0.9, P less than 0.05). Ultrastructurally a decrease in the number of exocytotic dense-cored vesicle (DCV) profiles at the level of the NEB basal epithelial cell membrane was found on the stimulated side (0.32 +/- 0.10 vs. 0.45 +/- 0.16 DCV/micron of basal epithelial cell membrane, n = 8, P less than 0.05). Section of the left vagus nerve without electrical stimulation affected neither the fluorescence intensity nor the number of exocytotic DCV profiles. In animals with supranodosal or infranodosal chronic vagotomy the observed effects of unilateral vagal stimulation were no longer present. We conclude that 1) vagal stimulation increases the serotonin content of NEB; 2) it decreases the number of exocytotic DCV profiles; 3) this effect depends on the amplitude of the stimulus; 4) it is obtained through efferent vagal fibers; 5) these results are the opposite of the effects seen after exposing normal NEB to acute hypoxia; and 6) these physiological experiments corroborate a vagal innervation of NEB, which may play an important role in modulating the sensitivity and reaction of NEB to various stimuli.