The early effects of truncal bilateral vagotomy were studied in dogs by means of electron microscopy from three hours to four weeks after denervation. Vagotomy causes degeneration of some nerve fibers, vasoconstriction with resulting transitory ischemia and aberrations in the substructure of secretory cells in the gastric mucosa. Nerve degeneration causes a cholinergic deficit which is responsible for the clinical effects of vagotomy. Ischemia is most probably the cause of alterations observed in chief and parietal cells, but it does not account for the long-term reduction of functional ability of the gastric mucosa after vagotomy.