Clinical observations and limited animal experiments have suggested that gastrointestinal motility is suppressed during pregnancy. We therefore compared isometric contractions of colon and ileal circular muscle in response to carbachol (10(-8) to 10(-4) M). Data was analyzed by comparing mean maximal tension, dose-response curves, and EC50 values for tissue from the two groups of animals. Circular muscle from proximal colon, distal colon, and ileum in pregnant animals developed less tension in response to carbachol than did tissue from non-pregnant controls. Dose-response curves in the pregnant groups were depressed, when compared with non-pregnant groups, at concentrations of 10(-6) M and greater. Sensitivity of the muscle to cholinergic stimulation, as measured by EC50 values, was similar in the ileum and proximal colon but increased slightly (p less than 0.05), by a factor of approximately 2, for distal colonic muscle from pregnant animals. Assuming that circular muscle contractions are primarily responsible for mixing and propulsion in the gut, this reduction in responsiveness to excitatory cholinergic stimulation is consistent with the concept of pregnancy-related suppression of gastrointestinal motility.