Two studies were performed to determine the effects of resistant starch (RS) on body weight and fat. A 2x2 factorial design was used in both studies, and results were considered significant when p<0.05 for both studies. The first study examined the effects of RS in a high fat diet (44.8% of energy) on weight, fat, peptide-YY (PYY) levels, and cecal pH in male Sprague-Dawley rats. Rats were fed a low fat energy control diet for one week prior to diet treatment. On week two, rats were blocked by weight and fed one of the following diets for 12 weeks (n=10): low fat, energy control (LFEC); LF resistant starch (LFRS); high fat, energy control (HFEC); or high fat resistant starch (HFRS). RS did not lower weight or fat with either the HF or LF diets. RS consumption resulted in greater full and empty cecal weights, and a lower pH for the LFRS diet. This data indicate fermentation, even though weight and fat loss did not occur. This is contrary to previous reports with RS, which has been shown to decrease body fat compared to controls. The second study examined the effects of RS on the weight, fat, PYY levels, and glucagon-like peptide-1 (GLP-1) levels in female Sprague-Dawley rats. Ovariectomized (OVX) rats were used to represent rats prone to gaining weight, and sham rats represented normal rats. Rats were assigned to one of four groups (n=10): OVEC, OVRS, SHEC, or SHRS. Rats were fed the EC diet for 6 weeks prior to diet treatment to gain weight after surgery, and then blocked by weight and fat into diet treatment groups, and spent 13 weeks on treatment diets. Energy intake, total gastrointestinal weight, large intestine/cecum weight, and small intestine weight were all higher in RS fed rats relative to EC fed rats. Mesenteric, ovarian, perirenal, retroperitoneal, and total fat pads were lower in RS rats relative to EC rats. Although RS was not effective in lowering body weight or body fat in the first study, the data indicates that resistant starch may lower body weight and fat in postmenopausal women.