Abstract: Melatonin receptors have previously been shown to elicit cellular signaling through the hematopoietic-specific G protein, G16. In the present study, we show that this functional coupling elicited biphasic stimulatory phosphorylation on STAT3 in recombinant MT1/Gα16 cells and native Jurkat T cells (endogenously expressing MT1 and Gα16), with maximal Ser727 phosphorylation occurring at 15 min, while marked Tyr705 phosphorylation became detectable only upon agonist treatment for 4 hr or more. By employing signal transducer and activator of transcription 3 (STAT3) phosphorylation-resistant mutants (STAT3-Y705F and STAT3-S727A), we further showed that the receptor-mediated STAT3 phosphorylations at Ser727 and Tyr705 were independent of each other. Results obtained from fractionation of 2-IMT-induced cells revealed that the Ser727 and Tyr705 phosphorylations were spatially distinct, with the former mainly situated in mitochondria and cytosol, while the latter was predominantly located in the nucleus. Further experiments revealed that the agonist-induced STAT3 phosphorylation at Tyr705 was significantly suppressed by pretreatment with cycloheximide (a ribosome inhibitor), suggesting that de novo protein synthesis might play a critical role for this response. Using conditioned media obtained from 2-IMT-treated MT1/Gα16 cells, multiplex immunoassays revealed that prolonged agonist treatment led to elevated productions of IL-6, GM-CSF and CXCL-8. Antibody against IL-6, but not those for GM-CSF and CXCL-8, effectively abolished the agonist-induced STAT3 Tyr705 phosphorylation, suggesting the involvement of IL-6 in melatonin receptor-mediated STAT3 activation. Our results demonstrate that melatonin receptor/Gα16 coupling is capable of triggering the production of cytokines including IL-6, and this autocrine loop may account for the subsequent STAT3 phosphorylation at Tyr705.