Pyruvate-dehydrogenase, an enzymatic mitochondrial complex, exists in both inactive and active forms, insulin being the regulating factor of the transformation of a latter into the former. The basal (PDHb) and total (PDHt) activity of this enzyme in adipose tissue mitochondria from obese hyperinsulinemic humans has been found equal to 1014 +/- 459 SD mU respectively. These values are 250% higher than those found in normal subjects (403 +/- 76 SD and 575 +/- 142 SD respectively). Both in normal and obese subjects the PDHb/PDHt percent ratio was equal to about 70. These results show that insulin, undoubtedly hyperactive in obesity, by activating PDH can induce a major synthesis of fat, a high caloric density tissue.