The hypothesis is that R108512 dose dependently accelerates colonic transit in patients with functional constipation or constipation-predominant irritable bowel syndrome. The specific aim of this study is to measure gastric, small bowel, and colonic transit in 40 patients with functional constipation or constipation-predominant irritable bowel syndrome randomized to placebo, 0.5, 2.0, or 4.0 mg per day as a single dose of R108512.