Transit time, as estimated from the passage through the gut of an isotope (131I)-containing capsule, was studied in 10 constipated, geriatric in-patients on a bulk laxative regimen and during treatment with wheat bran. The patients received a conventional bulk laxative (Vi-Siblin, 6 g twice daily) during a period of 8 weeks. The bulk laxative was then replaced by bran (10 g twice daily) during the following 8-week period. Measurements of transit time were taken after 4 and 6 weeks on each regimen. The mean transit time was 126 h for the bulk laxative regimen and 89 h for the bran treatment, the difference being statistically significant. The decrease in transit time for the bran treatment was essentially due to a faster passage through the rectosigmoid part of the bowel. Less additional laxative therapy was needed during bran treatment than during the bulk laxative treatment. The favourable properties of bran may tentatively be explained by the effects remaining in the distal part of the colon. A slight but significant lowering of the serum calcium level and a significant increase of the total iron-binding capacity of the serum were found after 7 weeks of bran administration, whereas no change was observed after 42 weeks. No significant change was demonstrated in serum iron during bran treatment.