We examined the role of resistant protein and peptides in promoting cecal butyrate production in rats fed rapidly fermentable carbohydrates. Rats were fed diets containing raw potato starch (RPS, 200 g/kg diet) or fructooligosaccharide (FOS, 60 g/kg diet) with casein, soy or rice protein (250 g/kg diet) for 13 d. In rats fed RPS with casein, the major cecal organic acid was acetate (441 micromol), but lactate and succinate were also found in considerable amounts (324 micromol). Succinate was the major cecal organic acid (235 micromol) in rats fed FOS with casein. When rice protein was fed with RPS, the contribution of lactate was significantly lower and that of propionate tended to be higher (P < 0.1) than in rats fed casein. In rats fed rice protein with FOS, cecal butyrate and acetate were greater and cecal succinate was lower than in rats fed casein with FOS (P < 0.05). Despite the similar amounts of undigested protein in rice and soy proteins, soy protein did not similarly affect cecal butyrate in rats fed FOS or RPS. In another experiment, rats were fed diets containing high amylose cornstarch (HAS, 200 g/kg diet) with casein, casein + oligo-L-methionine (OM, 3 g/kg diet), soy protein, soy protein + OM (3 g/kg diet) or rice protein (250 g/kg diet) for 10 d. OM (digestibility, 31%) was substituted for the same amount of casein. Rats fed rice protein had greater cecal butyrate than rats fed casein (P < 0.05). OM supplementation to casein or soy protein increased cecal butyrate compared with rats fed casein or soy protein alone (P < 0.05). These data support our hypothesis that resistant protein and peptides promote cecal butyrate production and suggest that the differing potency of rice and soy proteins in promoting cecal butyrate production might be explained in part by the different amino acid composition of resistant protein.