To better understand the physiology of colonic gas production, each flatus passage of 16 subjects over a 4-h period was analyzed by gas chromatography for N2, O2, H2, CO2, CH4, and for odoriferous sulfur-containing gases. Appreciable intraindividual and enormous interindividual variability was observed, indicating that each gas passage reflected the interaction of highly variable liberation and/or removal mechanisms. The predominant flatus gas was CO2, H2, and N2 in seven, six, and three subjects, respectively. Gases produced intraluminally (H2, CO2, and CH4) comprised approximately 74% of flatus, and rapid CO2 and H2 productions were responsible for high passage rates. A positive correlation between flatus H2 and CO2 suggested that CO2, like H2, mainly was a bacterial product. Whereas methanogens and H2S-producing bacteria usually are mutually exclusive in feces, CH4 and H2S did not negatively correlate, indicating coexistence of both organisms in the colon. We conclude that analysis of flatus composition provides a novel means of assessing colonic physiology, particularly ongoing bacterial metabolism throughout the unperturbed colon.