The effects of varying intake of dietary fiber content on left colon intraluminal pressure, electrical spike discharge activity, and contractions of circular muscle were measured in stump-tailed monkeys. Animals were preconditioned to sit in restraint chairs and then had the sigmoid colon instrumented with an extraluminal strain gage force transducer, a bipolar serosal electrode and a Thomas cannula to provide access for determining luminal pressure with a perfused, open-tip catheter. Diets containing 0, 5, 10, 15, and 20 g of dietary fiber daily were fed in varying sequences for periods of 3 wk. Observations were made twice during the last week of each diet. Fecal weight increased in animals on a high fiber diet, but changing dietary fiber content did not alter the duration of spike discharge activity and produced only a minimal change in the contraciton frequency of colon circular muscle. Colon intraluminal pressure was increased significantly by feeding low-fiber diets, but intraluminal pressure responses did not always correlate with the myoelectrical activity of the colon circular muscle.