Eight obese female patients were studied over a period of 15 days whilst on 300 kcal diet. Serum levels of thyroxine and free throxine index were not altered significantly by semistarvation. A TRH test performed before and after the diet showed no appreciable change. Weight loss was intially rapid but later slowed despite good patients compliance. Serum concentrations of T 3 and reverse T 3 (rT3) early decreased (p less than 0.01) and increased (p less than 0.05) respectively, but returned towards control levels even before discontinuation of semistarvation. There was a positive correlation between the percentage decrease in body weight and the percentage increase in serum rT 3 (p less than 0.001), and a negative correlation between decrease in body weight and decrease in serum T 3 (p less than 0.001). Our results do not suggest that the variations in serum triiodothyronines limit the weight loss; it is probable, on the contrary, that the weight loss promotes the observed variations in thyroid hormones by as yet unknown adaptive metabolic forces.