We studied the influence of hydrogen tension (PH2) and methanogenesis on H2 production and consumption by human fecal bacteria. Hydrogen consumption varied directly with PH2, and methanogenic feces consumed H2 far more rapidly than did nonmethanogenic feces. At low PH2, H2 production greatly exceeded consumption and there was negligible accumulation of the products of H2 catabolism, methane and sulfide. Thus, incubation at low PH2 allowed the first reported measurements of absolute as opposed to net H2 production. Feces incubated at high and intermediate PH2 had a net H2 production of only 1/900 and 1/64 of absolute production. Glucose fermentation by fecal bacteria yielded an absolute H2 production of 80 ml/g, a value far in excess of that excreted by volunteers ingesting lactulose. We conclude that most H2 produced by colonic bacteria is consumed and methanogenesis and fecal stirring (via its influence on fecal PH2) are critical determinants of H2 consumption and, hence, net H2 production. Study of fecal samples from four subjects with low breath H2 excretion after lactulose showed that absolute H2 production was normal, and the low H2 excretion apparently reflected increased consumption due to rapid methanogenesis (two subjects) and decreased luminal stirring (two subjects).