Studies were designed to examine the local IGF-1 system in adipose tissue development. The first experiment gave direct evidence of IGF-1 receptors on preadipocytes in primary cultures using fluorescence double-label immunocytochemistry verifying that IGF-1 could directly act on preadipocytes. The second study compared IGF-1 and IGF-1 receptor mRNA expression in white adipose tissues of 12-wk-old genetically obese Zucker rats with that of lean controls using relative quantitative RTPCR. Fat depot specific differences in IGF-1 and IGF-1 receptor expression were detected, with expression generally higher in tissues from the obese as compared with the lean rats. The IGF-1 mRNA levels were higher than that of IGF-1 receptor within the same depot. In vivo the regulation of IGF-1 and IGF-1 receptor in adipose tissue of obese rats is depot-specific and happens at the transcriptional level. The activation of local IGF- 1 system in adipose is mainly through the increase of IGF-1 receptor levels in obesity.