Adult rats were subjected to either a 90 to 95 percent jejunoileal bypass or a sham operation and were sacrificed 35 days after surgery. Rats with jejunoileal bypass lost 33 percent of their original weight, whereas the sham operated rats gained 14 percent. Food intake per 100 g body weight was significantly increased between postoperative days 14 and 35 in the jejunoileal bypass rats. Levels of tryptophan were significantly reduced in the cortex, hypothalamus, striatum, hippocampus, mesencephalon, diencephalon, pons-oblongata, and cerebellum, whereas serotonin concentrations were lowered in the diencephalon, pons-medulla, and cerebellum in jejunoileal rats compared with control rats. Levels of 5-HIAA were reduced in the hypothalamus, cortex, mesencephalon, and diencephalon. In the plasma of bypassed rats, concentrations of valine, leucine, isoleucine, tryptophan, methionine, threonine, and tyrosine were significantly lower than in the control rats. In the cerebral cortex, levels of phenylalanine, tyrosine, histidine, and glutamine were increased. The results suggest involvement of indoleamine metabolism in disrupted eating after jejunoileal bypass. The elevated brain levels of glutamine, phenylalanine, tyrosine, and histidine resemble similar changes seen after portosystemic shunting in rats.