Mouth to caecum transit time (M.C.T.T.) was assessed in 41 subjects divided into 3 age groups (0-1 year, 1-4 years and 22-27 years) with the hydrogen breath test (H.B.T.) using lactulose as the hydrogen source (250 mg/kg; minimal dose 1.5 g in a 10% aqueous solution). The M.C.T.T. remained constant from the second month of age. In children under 1 year of age a high percentage of non-responders was found compared to the older age-groups. In 8 adults two test-substrates, pure lactulose (Legendal, L) and a mixture of lactulose, lactose and galactose (Duphalac, D) were compared. There was no significant difference in M.C.T.T., mean hydrogen production and clinical symptoms using D or L as substrate. We conclude that the H.B.T. using a relative dosage is a suitable test for children, but under the age of 1 year it is less useful because of the high percentage of non-responders. Because of the relation between body height and small bowel length the results suggest an increase in the velocity of the intestinal content with age.