Fifteen grossly obese patients were studied before and 6-8 months after gastroplasty. Their mean body weight decreased by 30% (from 127 +/- 13 to 97 +/- 14 kg, mean +/- SD). The preoperative hyperinsulinemia, hyperglucosemia and hyperglucagonemia were significantly reduced at follow up. Liproprotein lipase activity, measured in post-heparin plasma, was slightly reduced and did not change after weight reduction. Variables reflecting thyroid function were within the reference ranges; small but statistically significant reductions in serum thyroxine and reverse triiodothyronine levels were recorded. Adipocyte heat production, reflecting total cellular metabolic activity and registered by microcalorimetry, was significantly decreased before surgery (by about 60 per cent when expressed per g tissue and by about 20 per cent when expressed per cell) and increased significantly at follow-up; expressed per cell, the heat production was normalized, but expressed per g tissue the values were still about 15 per cent before those of a control group. The results indicate that the reduced adipocyte heat production in obese individuals is a consequent rather than a cause of severe obesity.