Histochemical study of the distribution of cholinesterases in the cat medulla oblongata reveals that all neurones in the dorsal motor nucleus of the vagus (DMV) contain true cholinesterase (AChE) but, while most contain this enzyme alone, a small proportion of cells contain pseudocholinesterase (BuChE) as well. Cervical vagotomy affects the two types of cell to different degrees of severity. The BuChE-containing neurones lose their enzyme completely within 2-3 weeks and they atrophy and disappear as a result of the operation. On the other hand, the reaction is less severe and is reversible in those cells containing AChE only. Vagotomy also causes reduction of AChE and BuChE staining in the nearby area subpostrema; the depletion here is pronounced at 2-3 weeks and recovery occurs within the year. These findings suggest that some cells in the area subpostrema project peripherally via the vagus and that the area is part of the vagal nuclear complex. Moreover, capilllaries in the area contain AChE and BuChE in the endothelial lining and this is one of the few areas of the cat hindbrain to exhibit such vascular enzyme activity. The ependyma of the area postrema, which overlies the area subpostrema, is heavily stained for BuChE but this is unaffected by vagotomy.