In 10 cats and 10 opossums, serial transverse strips of muscularis propria 2 mm wide, from about 1.5 cm above to about 1.5 cm below the ileocecal mucosal junction, were separately stretched to discover the relationship between length and tension and were separately subjected to electrical field stimulation. In both species, ileal strips showed stimulus relaxations, sometimes accompanied by contractions after the stimulus trains (off responses), whereas colonic strips generally did not. These responses were abolished by tetrodotoxin, 3 times 10-7 M in the opossum, 3 times 10-6 M in the cat. Length-tension slopes were greatest just above the ileocecal junction where stimulus relaxation occurred. Tetrodotoxin at 3 times 10-7 M raised basal tension in ileal strips in both species and in colonic strips only in the cat. Muscle of the ileocecal junction resembles that of the esophagogastric and gastroduodenal junctions in showing a relatively high degree of resistance to stretch and prominent neurogenic stimulus relaxation. However, the ileocecal junction, in contrast, also possesses a tonic inhibitory innervation.