Our subject T, who appeared to be a suitable candidate for an assessment of the effects of oligosaccharides, was administrated fructooligosaccharide (FOS) or lactulose and his symptoms, fecal microflora, and composition of rectal gas were investigated. The subject experienced no symptoms in the gut at low dose of FOS. At high dose of FOS, tympanites was observed in the ascending and the transverse colon. By lactulose administration, there were no tympanites, but frequent defecation, rectal gas and loose stool were noted. The number and occupation percentage of fecal bifidobacteria increased by higher doses of FOS (10^9.1 to 10^10.8, 2.9 to 66.2%) and lactulose (10^8.7 to 10^10.5, 0.5 to 25.2%). The composition of rectal gas varied even among samples collected in succession and no clear relationship was observed between the composition of gas and dose. Based on these results, the course of tympanites is discussed.