Behavioral modification of colonic function. Can constipation be learned? We challenged the two hypotheses: first, that defecation can be suppressed for an extended time, and second, if so, that this has an effect on upper colonic motility. Thus we studied 12 male volunteers with conditions of identical nutrition and patterns of physical activity over a two-week period, where one week with normal defecation and one week with voluntary prolonged suppression of defecation followed each other in randomized order. Frequencies of defecation, stool weights, total and segmental colonic transit times (using radiopaque markers) were compared. Frequency of defecations and stool weights were lower during suppressed defecation [8.9 +/- 0.66 vs 3.7 +/- 0.41 (mean +/- SE) bowel movements per week, P = 0.003, and 1.30 +/- 0.09 vs 0.98 +/- 0.13 kg/week, P = 0.01]. Total transit times were increased from 28.8 +/- 4.4 to 53.1 +/- 4.3 hr, P = 0.004. Segmental transit times were increased in the rectosigmoid (from 8.83 +/- 3.6 to 32.1 +/- 5.6 hr, P = 0.04) and right hemicolon (from 14.5 +/- 0.9 hr to 19.7 +/- 1.5 hr, P = 0.02) by suppression of defecation. We conclude that defecation habits may induce changes in colonic function such as those seen in constipation and that functional anorectal outlet obstruction may, probably by reflex mediation, affect the right colon. 