The abilities of dietary fibers to promote the excretion of intestinal fermentation gases were evaluated in five healty men. Flatus and breath gases were collected and analyzed during 3 days of each 9-day metabolic period. Responses to feeding xylan, pectin, cellulose and corn bran were compared to a fiber-free formula diet. Generally, hydrogen production increased throughout the day, whereas methane production remained more constant. Methane excretion was greater while consuming the xylan and pectin diets than while consuming the other diets. These two purified fibers also caused higher flatus volume, hydrogen and carbon dioxide excretion. Cellulose and corn bran generally resulted in breath and flatus gas excretion at levels equivalent to fiber-free diets. Considerable variation was noted in the response of subjects to an individual diet. However, in most cases 2-5 days of frequent and daily consumption of the diets was necessary to establish a relatively constant level of gas excretion. This time may represent the period of microbial and enzymatic adaptation to the new dietary constituents.