Hypocaloric dextrose administration results in a diminished minute ventilation, metabolic rate, and ventilatory responsivity to hypercapnea and hypoxia which is rapidly reversed by provision of amino acids in individuals of normal weight. This study compared the effects of peripheral intravenous dextrose and amino acid infusion on respiratory parameters and energy expenditure in 25 morbidly obese patients undergoing gastric bypass surgery. On the first postoperative day, respiratory rate increased (P less than 0.01) and tidal volume decreased (P less than 0.001) maintaining minute ventilation at slightly less than preoperative levels but on subsequent days minute ventilation exceeded baseline values (P less than 0.005) primarily by a sustained increased respiratory rate. Oxygen consumption (P less than 0.005), carbon dioxide production (P less than 0.05), and resting energy expenditure (P less than 0.01) all declined 25% from baseline values on the first postoperative day and subsequently reverted to preoperative values. The type of intravenous fluid had no effect on any of these parameters. A significant difference in respiratory quotient (P less than 0.05) was noted between the two intravenous fluid regimens attributable to the oxidation of the dextrose calories. The failure to detect a difference in metabolic rate or to stimulate respiration despite elevation of serum branched chain amino acids (P less than 0.0001) and ketone bodies (P less than 0.0001) with protein infusion does not suggest a role for nutrient manipulation of respiration in the postoperative care of the morbidly obese patient.