Loperamide was used to provide a source of opioid-induced constipation in healthy volunteers. Each volunteer took a sequence of three dose levels of loperamide. One of three laxatives was used to counterbalance the effect of loperamide and restore bowel function to what the individual considered normal at each stage before the dose of loperamide was increased. Lactulose, senna, and codanthrusate were selected as examples of a softening, a stimulant, and a combination laxative, respectively. Outcome measures were the doses of laxative used, stool form and frequency, ease of defecation, a rating scale of subjective bowel function, and the occurrence of adverse effects. Each laxative was taken by ten volunteers, and all proved capable of maintaining normal bowel function. A combination of stimulant and softening laxatives was most likely to maintain normal bowel function at the lowest dose and least adverse effects. The mean final dose of lactulose was excessive for use in ill patients. Senna was associated with significantly more adverse effects than the other laxatives, mainly abdominal pain (P < 0.001). This model of constipation may provide a standardized means of assaying the clinical effectiveness of oral laxatives.