Recently, apelin was characterised as a novel adipose-expressed factor which is upregulated in rodent and human obesity and influences cardiovascular function, as well as insulin secretion. To clarify expression and regulation of this adipokine, apelin mRNA was measured by quantitative real-time reverse transcription-polymerase chain reaction in mouse 3T3-L1 adipocytes after treatment with various hormones known to induce insulin resistance. Interestingly, apelin synthesis was significantly upregulated by growth hormone (GH) and insulin in these cells whereas TNFalpha and isoproterenol did not have any effect. Thus, 500 ng/ml GH acutely induced apelin mRNA by up to 4-fold in a time-dependent fashion with significant stimulation seen at concentrations as low as 5 ng/ml effector. Furthermore, apelin secretion was assessed by enzyme-linked immunoassay in mouse adipocytes. Here, secretion of this adipokine was induced 2.85-fold by GH. Studies using pharmacological inhibitors suggested that the positive effect of GH on apelin mRNA synthesis is at least in part mediated by janus kinase 2 and phosphatidylinositol 3-kinase. Taken together, our results show a significant induction of apelin mRNA synthesis and protein secretion by GH.