A decreased mobilization of triglycerides may contribute to fat accumulation in adipocytes, leading to obesity. However, this hypothesis remains to be proven. In this study, epinephrine-induced lipid mobilization was investigated in vivo in nine markedly obese children (160+/-5% ideal body weight) aged 12.1+/-0.1 yr during the dynamic phase of fat deposition, compared with six age-matched nonobese children. As an in vivo index of lipolysis, we measured glycerol flux using a nonradioactive tracer dilution approach, and plasma free fatty acid concentrations. In the basal state, the obese children had a 30% lower rate of glycerol release per unit fat mass than the lean children. To study the regulation of lipolysis, epinephrine was infused stepwise at fixed doses of 0.75 and then 1. 50 microg/min in both groups. In lean children, glycerol flux and plasma free fatty acid increased to an average of 249-246% of basal values, respectively, in response to a mean plasma epinephrine of 396+/-41 pg/ml. The corresponding increase was only 55-72% in the obese children, although their mean plasma epinephrine reached 606+/-68 pg/ml. All obese and nonobese children, except an Arg64Trp heterozygote, were homozygotes for Trp at position 64 of their beta3-adrenoreceptor. The resistance of lipolysis to epinephrine showed no relationship with the Arg64 polymorphism of the beta3-adrenoreceptor gene. In summary, in vivo lipolysis, which mostly reflects the mobilization of lipid stores from subcutaneous adipose tissue, shows a decreased sensitivity to epinephrine in childhood onset obesity. Since our study was carried out in obese children during the dynamic phase of fat accumulation, the observed resistance to catecholamines might possibly be causative rather than the result of obesity.