Native lactulose was used in breath hydrogen test (BHT) to detect small bowel transit time (SBTT) in normal persons (34 cases) and patients with functional diarrhea (24 cases) and functional constipation (12 cases). The mean SBTT was 80.6 +/- 28.3 min, 58.4 +/- 42.2 min and 104.2 +/- 21.0 min respectively. When the mean SBTT in the patient groups was compared with that in the normal group, there was statistically significant difference (P less than 0.05 and 0.01 respectively). The results showed that native lactulose produces hydrogen successfully (94.3%). Few side-effects were found, if a dose of 10g was used.