Human adipose tissue exhibits marked age-dependent differences in morphology and regulation of lipolysis. The mechanism behind this phenomenon is poorly characterized. The aim of the present investigation was to evaluate the antilipolytic effect of insulin and the expression of insulin receptor mRNA in human adipose tissue during development. To study the antilipolytic effect of insulin, lipolysis was induced with submaximal effective concentrations of isoprenaline or forskolin in adipocytes from five infants below 2 mo of age, five infants above 2 mo of age, six children, and five adults. Isoprenaline was equally effective in stimulating lipolysis in all age groups, whereas forskolin was significantly more effective in adipocytes isolated from adults than from infants and children. Increasing the concentration of insulin during the incubation allowed construction of dose-response curves of insulin inhibition of lipolysis. Maximal inhibition was observed with 30 microU/mL of insulin in the presence of either isoprenaline or forskolin. No differences in the maximal antilipolytic effect of insulin or in the insulin sensitivity expressed as ED50 were observed. Furthermore, the expression of insulin receptor mRNA in adipocytes did not differ from six infants, five children, and six adults as determined with a solution hybridization RNase protection assay. Thus, age-dependent variations in the regulation of lipolysis do not appear to be accompanied by variations in insulin action and insulin receptor gene expression in isolated adipocytes.