Pulmonary H2 excretion was used to quantitate the small bowel transit time in man. This technique is based on the observation that H2 is produced when carbohydrate is fermented by colonic bacteria and that this H2 production is reflected by a concomitant increase in breath H2 excretion. The time, therefore, between ingestion of the unabsorbable disaccharide, lactulose, and the rise in breath H2 represents the small intestinal transit time of the head of the lactulose load as it passes through the gut. Following ingestion of a mixture of polyethylene glycol (PEG) and lactulose by 9 subjects, transit time measured by H2 excretion correlated closely with the simultaneously determined time for PEG to reach the distal ileum (r equals 0.97). The ileal appearance of PEG preceded the rise in H2 excretion by a mean of 7.6 minutes. Transit time of 19 Gm. of lactulose in 40 healthy subjects averaged 72 minutes (range 25 to 118). Studies repeated 3 to 5 times in 6 subjects showed good individual reproducibility with subsequent measurements differing from initial by a mean of plus or minus 14 per cent. There was an inverse relation between transit time and dose of lactulose ingested by 9 subjects with 5, 10 and 20 Gm. of lactulose having mean transit times of 128 plus or minus 19, 94 plus or minus 15, and 40 plus or minus 8 (S$M.) minutes, respectively. This technique appears to provide a simple, safe, and noninvasive means of studying small bowel transit time in man.