In this study designed to investigate the nutritional state induced by biliopancreatic bypass in the rat, the pancreatico-biliary secretions were diverted via the duodenum and jejunum into the distal ileum, the remaining intestine being directly anastomosed to the stomach after antrectomy. Bypassed animals lost weight: it was only 56 percent of that of controls after 36 days and death by cachexia resulted within two months of the procedure. The reduced food intake (16 percent less than control) at the 36th postoperative day cannot by itself explain the weight loss, since pair-fed rat weights did not differ statistically from controls at 36 days. Protein-energy malabsorption occurred: drops in serum protein concentration (25 percent less than control), triglycerides (40 percent less) and total cholesterol (28 percent less) were recorded from the 12th postoperative day on. Biliopancreatic bypass may be an adequate procedure of treatment for morbid obesity simultaneously aggravated by metabolic disorders.