The musculature of the pylorus of the guinea-pig consists of a conspicuous ring of circular musculature. On one side, this tissue is apposed to the circular muscle layer of the duodenum, a complete septum of connective tissue intervening between the two muscles. On the other side, it is in continuity with the circular musculature of the gastric antrum. Bundles of longitudinal muscle, running close to the submucosa of the antrum, form a loop and contribute to the circular musculature of the pylorus. The subserosal longitudinal muscle of the antrum continues into the pylorus and the duodenum. The density of innervation, i.e. number of bundles, number of axons and percentage of varicosities per unit sectional area or per number of muscle cell profiles (estimated on large photographic montages of transversely sectioned muscles) is higher in the pylorus than in the duodenum, and is lowest in the antrum. In the duodenum nerve bundles occur in both the circular and the longitudinal muscle layer; the majority of axons and varicosities are situated between the bulk of the circular muscle and a layer of special muscle cells adjacent to the submucosa.