Background & Aims: Opiate bowel dysfunction is a significant clinical problem. Our aim was to evaluate the ability of a peripheral μ-opioid antagonist, alvimopan, to reverse the effect of codeine on gastric, small-bowel, and colonic transit time in healthy volunteers. Methods: Seventy-four healthy participants (43 women) were randomized in a double-blind, placebo-controlled manner to 1 of 4 groups: alvimopan 12 mg twice daily in the presence and absence of codeine sulfate 30 mg 4 times/day, or codeine or placebo alone. Gastric emptying, small-bowel, and colonic transit were measured by scintigraphy using a 99m-labeled technetium egg meal and 111-labeled indium charcoal delivered to the proximal colon via a delayed-release capsule. The primary end points for colonic transit were geometric center of the colonic counts at 24 hours and time for 50% ascending colon emptying. Analysis of covariance was used to assess the significance of the primary and secondary end points. Results: Codeine delayed gastric, small-bowel, proximal, and overall colonic transit ( P < .05). Alvimopan reversed codeine’s effect on small bowel and colon (ascending colon and overall colonic transit). Alvimopan also accelerated overall colonic transit compared with placebo. Thus, the mean colonic geometric center at 24 hours was 2.33 with placebo/placebo, 3.25 with alvimopan/placebo ( P < .05), 1.5 with placebo/codeine ( P < .05), and 2.63 with alvimopan/codeine. Alvimopan did not reverse codeine’s delay of gastric emptying. Conclusions: Alvimopan reverses codeine’s inhibitory effect on small-bowel and colon transit and has potential for treatment of opiate bowel dysfunction. Alvimopan alone accelerates colonic transit, suggesting that μ-opiate mechanisms participate in the physiologic control of colonic transit.