We have recently demonstrated that STATs 1, 3, 5A, 5B, and 6 are expressed in adipocytes. Using the 3T3-L1 cell line, we have examined the activation of all adipocyte expressed STATs by 23 different factors which are either potent regulators of adipocyte gene expression or known STAT activators in other cell types. STAT activation was assessed by examining nuclear translocation and tyrosine phosphorylation in serum deprived, fully differentiated 3T3-L1 adipocytes. Unlike other adipocyte-expressed STATs, STAT 5A was present in the nucleus under basal conditions. Of all the activators examined, only growth hormone was capable of causing STATs 5A and 5B to translocate to the nucleus. None of the activators were capable of affecting the cellular distribution of STAT 6. Furthermore, our results indicate that there is a quantitative activation of STATs 1 and 3 by LIF, OSM, and IFN-gamma as measured by both nuclear translocation and tyrosine phosphorylation in whole cell extracts. IFN-gamma is a potent activator of STAT 1 and a weaker activator of STAT 3, whereas LIF and OSM are potent activators of STAT 3 and weaker activators of STAT 1. These studies demonstrate that STAT activation in adipocytes is highly specific, quantitative, and distinct.