The expression and secretion of IGF-I and IGFBP-3 were investigated in cultured human preadipocytes and in in vitro differentiated adipocytes derived from human subcutaneous adipose tissue under chemically defined culture conditions. Human preadipocytes expressed mRNAs for IGF-I and IGFBP-3 and secreted the corresponding proteins into the culture medium as measured by sensitive radioimmunoassays. In human adipocytes; specific mRNA-expression was comparable to that found in preadipocytes, but IGF-I secretion was increased 10-fold (3.87 +/- 0.69 vs. 0.41 +/- 0.11 ng/ml/10(6) cells/48 hrs, p < 0.05) and IGFBP-3 secretion 2.5-fold (7.34+/-1.15 vs. 3.27+/-0.38 ng/ml/10(6) cells/48 hrs, p<0.05) in the presence of adipogenic medium probably resulting in an increase of unbound IGF-I. Under serum-free, chemically defined conditions human growth hormone (hGH) and insulin were found to be positive regulators and cortisol was found to be a negative regulator of IGF-I and IGFBP-3 secretion in preadipocytes. In cultured human adipocytes, hGH showed no effect on IGF-I and IGFBP-3 secretion, whereas insulin stimulated and cortisol inhibited the secretion of both proteins. We conclude that IGF-I and IGFBP-3 may not only exert their actions in human adipose tissue via circulation, but also in an auto/paracrine way.