Pyruvate dehydrogenase (PDH) has low activity in the circulating lymphocytes (CL) of obese adolescents and adults. In vitro , it is unresponsive to insulin at 5 μ -units/ml and is activated at 50 μ -units/ml, in contrast with activation and inhibition respectively at these concentrations in CL from controls. These changes are seen as being indicative of a molecular disorder underlying insulin resistance. The aims of the present study were to determine whether a substantial enhancement of blood insulin levels restores the PDH activity in CL from obese adolescents and abolishes the in vitro alterations, and whether PDH activity and indices of insulin resistance are correlated. Six obese adolescents and six normal-weight controls underwent a 4h frequently sampled intravenous glucose test with minimal model analysis, to bring about a sharp rise in blood insulin and provide a reliable index of insulin sensitivity (S I ). PDH activity was evaluated in CL obtained from blood samples at set times before and after their exposure to insulin in vitro . Insulin levels rose in all subjects in the first 10min, although to a much greater extent in the obese group, and then decreased until the end of the test (240min; t 240 ). PDH activity in CL paralleled the insulin pattern in the control subjects, whereas in the obese subjects it was below normal 3min before the start of the test ( t -3 ), but rose significantly throughout the test. PDH responses in vitro to insulin in CL taken from the control subjects at t -3 and t 240 and in CL taken from the obese subjects at t -3 were as reported above, but were normal (i.e. the same as in control CL) in CL taken from the obese subjects at t 240 . Baseline PDH activity was inversely correlated with body mass index and with fasting insulin, and directly correlated with S I . These results show that a brief and sharp enhancement of blood insulin overcomes derangements in PDH that reflect systemic insulin resistance in obese adolescents.