Background/Aim: Bariatric surgery Roux-en-Y gastric bypass (RYGB) has been designed to limit food intake by creating small gastric pouch and to reduce nutrient absorption by bypassing long-limb intestine. In fact, the stomach is also bypassed after RYGB. The aim of the present study was to elucidate the contribution of stomach bypass on body weight (BW) loss in rats. Methods: Male rats (Sprague-Dawley and Long-Evans) were used in a series of experiments including i) total gastrectomy (GX), ii) gastric bypass (GB), and iii) gastric + duodenal bypass (GDB). BW, body composition, bone mineral density (BMD), and bone mineral content (BMC) were followed up by dual energy X-ray absorptiometry (DXA). Eating activity, food and water consumptions, oxygen (VO2) and carbon dioxide (VCO2) consumptions, and locomotor activity were measured simultaneously by a computerized monitoring system. Serum gastrin and ghrelin levels were determined by radioimmunoassay. The stomachs were examined immunohistologically and the brains examined by Affymetrix GeneChip or in situ hybridisation analysis. Results: Rats (250 g BW) subjected to GX, GB or GDB gained 50-70% less than sham-operated rats during the postoperative course of 2 months. It was no difference between GB and GDB. One year follow-up by DXA showed that BW, BMD, BMC, lean and fat tissues were significantly lower in GX or GDB than sham-operated controls already after 1 month, and that GDB rats stopped growing after 6 months in contrast to the controls that gained continuously as a result of expansion of fat compartment. Long-Evans rats (600 g) subjected to GDB loss BW by 28% after 5 months, whereas sham-operated rats gained little. Food intake was not reduced rather than increased by 150% in GDB rats, which was resulted from frequently eating during both light and dark phases. VO2, VCO2 and spontaneous locomoter activity were slight higher in GDB rats than sham-operated controls. The serum concentration of ghrelin was reduced in GX rats but unchanged in GB or GDB rats, while hypogastrinemia developed in GX, GB or GDB rats. Gene expression profile in the whole brain and the gene expressions of neuropeptide Y, agouti-related peptide, anorexigenic pro-opiomelanocortin and melanin-concentrating hormone in the hypothalamus appeared to be unchanged after GX or GDB. Conclusions: The present study of rats suggests that 1) the body weight reduction after the gastric bypass surgeries is most likely due to the stomach exclusion; 2) the loss of body weight after gastric bypass surgery is not due to the reduced food intake; and 3) like gastrectomy, the gastric bypass may lead to the development of osteoporosis.