A method is described for the measurement of hydrogen in expired air, using an electrochemical detector. The apparatus is simple to use and sensitive. Its application is illustrated by studies of small bowel transit time made by measuring the time between oral ingestion of the unabsorbable carbohydrate lactulose and a rise in the concentration of hydrogen in expired air. In 20 control subjects transit time was 93.0 +/- 6.6 minutes, while in 16 patients with diarrhoea due to the irritable bowel syndrome it was 54.1 +/- 6.3 minutes (P less than 0.001), suggesting an abnormality in small intestinal motility in these patients. Loperamide, a potent antidiarrhoeal agent, increased transit time in 12 of these patients from 56.3 +/- 6.7 to 100.0 +/- 10.2 minutes (P less than 0.001).