Left cervical vagotomy increased NADPH-diaphorase (NADPH-d) histochemical staining in neuronal perikarya of the ipsilateral dorsal motor nucleus of the vagus (dmnX) and the rostral part of the nucleus ambiguus (nAmb). This effect appeared by 2 days, was maximal around 10 days, and declined by 30 days after vagotomy. Light and dark stained perikarya occurred in dmn X ipsilateral to the vagotomy which could not be explained on the basis of the biochemical or transmitter content of these neurons. It is unlikely that the increases of NADPH-d activity resulted from changes in cholinergic transmission since vagotomy is known to decrease cholinergic enzyme function. Since vagotomy increased both the glucose metabolic rate and NADPH-d staining of dmnX and nAmb in these experiments, it is more likely that these effects represent regenerative metabolic responses to axotomy.