The method described by Arhan et al. for measuring colon mean transit time requires plain films of the abdomen during 7 consecutive days (0.007 Gy). A new method is described for measuring the mean segmental transit time of radio opaque markers through the human colon which aims at decreasing ray exposure. While on a diet containing 20 g of bran, a dose of 20 markers was given to the subject for three consecutive days (D1, D2, D3) at the same hours. Each dose of markers was of different shape. A plain film of the abdomen was taken at D4, D7, D10 at the same time as the ingestion of the markers. The first X ray demonstrated the number of markers at 24, 48, 72 h simultaneously and so on from 96 to 216 h for the X ray taken at D7 and D10. Mean transit time in the right, left, rectosigmoid and the whole colon was measured in 22 volunteers and was respectively 6.9 h, 9.1 h, 18.4 h, 34.4 h. The upper limit of the mean transit time (m + 2 SD) was respectively 24 h, 30 h, 44 h, 67 h in the right, left, rectosigmoid and the whole colon. This method has been validated in 20 subjects (10 volunteers and 10 constipated patients) by comparing the mean segmental transit time with the mean segmental transit time measured by the method of Arhan et al. This new method could allow to explore patients with idiopathic constipation more frequently and to better assess the treatment of constipation.