Psyllium has been reported to inhibit lactulose-induced colonic mass movements and to benefit patients with irritable bowel syndrome, improving both constipation and diarrhea. Our aim was to define how psyllium modified the whole-gut transit of a radiolabeled lactulose-containing test meal by using gamma scintigraphy. Eight subjects participated in a randomized crossover study comparing gastric emptying and small bowel and colonic transit after consumption of 20 mL lactulose three times daily with or without 3.5 g psyllium three times daily. Psyllium significantly delayed gastric emptying: the time to 50% emptying increased from a control value of 69 +/- 9 to 87 +/- 11 min (mean +/- SEM; P < 0.05, n = 8). Small bowel transit was unaltered. However, progression through the colon was delayed with an increase in the percentage of the dose at 24 h in the ascending (control group: 2 +/- 3%, psyllium group: 11 +/- 8%; P < 0.02) and transverse colon (control group: 5 +/- 12%, psyllium group: 21 +/- 14%) with correspondingly less in the descending colon. Although the time for 50% of the isotope to reach the colon was not significantly different with psyllium, psyllium significantly delayed the rise in breath-hydrogen concentrations, which reached 50% of their peak at 217 +/- 34 min compared with control values of 155 +/- 27 min (P < 0.05). Psyllium delays gastric emptying, probably by increasing meal viscosity, and reduces the acceleration of colon transit, possibly by delaying the production of gaseous fermentation products.