Technics employing intestinal infusions of gas were used to study H2 production in the human intestine. The volume of H2 in the bowel of 10 normal subjects varied from 0.06 to 29 ml. H2 production, which averaged 0.24 ml per minute in the fasting state, sharply increased after intestinal instillation of lactose to a mean peak rate of 1.6 ml per minute. Ingestion of food also increased H2 production by seven-fold to 30-fold. In the normal intestine, more than 99 per cent of H2 production was colonic, but small-bowel production was increased in a patient with excessive numbers of small-bowel bacteria. H2 production in man is primarily dependent upon the delivery of ingested, fermentable substrates to an abundant intestinal flora that normally is present only in the colon. A mean of 14 per cent of the total H2 production was excreted by the lungs, and rates of breath H2 excretion and H2 production correlated well (r = 0.94). Respiratory H2 excretion can therefore be used as an indicator of intestinal H2 production.