Weight loss, alterations in basal metabolic rate, and utilization of body fat, carbohydrate, and protein substrates were studied in nine patients before operation and 3 and 12 months after gastric partitioning operation for morbid obesity. Respiratory oxygen consumption and carbon dioxide excretion measurements were taken three consecutive mornings by the open circuit-Scholander technique. Measurements of urine urea nitrogen were made from 24-hour urine collections. Basal metabolic rate and utilization of fat, carbohydrate, and protein were calculated in kilocalories per minute by indirect calorimetry. Initial body weight was 124.5 +/- 19.0 kg (mean +/- SD). The weight losses between measurements at months 0 and 3 and at months 3 and 12 were 20.8 +/- 4.6 kg and 2.7 +/- 8.4 kg, respectively. Total weight loss between months 0 and 12 was 23.5 +/- 8.3 kg (19.3% +/- 7.4%). At 3 months the fraction of basal metabolic rate contributed by carbohydrate (p less than 0.05) and protein (p less than 0.01) utilization decreased significantly, while that contributed by fat increased (p less than 0.05). Between months 0 and 12 there was no significant difference in protein or carbohydrate utilization, but fat utilization increased (p less than 0.10). CONCLUSIONS Gastric partitioning operation resulted in an initial rapid body weight loss over 3 months with a sustained reduction over 1 year; there was a metabolic utilization shift to fat with carbohydrate and protein sparing; no metabolic parameter was predictive of weight loss; and temporally, the rapid weight loss was paralleled by a significant metabolic utilization shift, and the sustained loss was paralleled by a stabilization of this shift.