The ultrastructure of the atrial myocardium in the monkey (Macaca fascicularis) was studied after bilateral cervical vagotomy and survival times of 100, 175 and 367 days. Changes were observed in the nucleus and the cytoplasm of the myocyte. Sequestered within the nuclei of the affected myocytes were cytoplasmic organelles and inclusions. In the late stages, there was a tendency towards condensation and margination of the heterochromatin. Changes in the cytoplasm included increased glycogen, mitochondrial degeneration and myofibrillar disorganisation and degeneration. There was increased collagen and mononuclear cell infiltration in the extracellular space in the later stages. This study has shown that the long term structural integrity of the atrial myocyte depends on an intact vagal innervation. The survival of the monkey after chronic bilateral vagotomy suggests that this nonhuman primate is a suitable model for functional studies of the parasympathetically denervated heart.