Growth hormone and prolactin are important growth factors for pancreatic β-cells. The effects exerted by these hormones on proliferation and on insulin synthesis and secretion in β-cells are largely mediated through the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) signaling pathway. Suppressors of cytokine signaling (SOCS) proteins are specific inhibitors of the JAK/STAT pathway acting through a negative-feedback loop. To investigate in vivo effects of SOCS-3 in growth hormone (GH)/prolactin signaling in β-cells we generated transgenic mice with β-cell-specific overexpression of SOCS-3. The relative β-cell proliferation and volume in the mice were measured by morphometry. β-Cell volume of transgenic female mice was reduced by over 30% compared with β-cell volume in wild-type female mice. Stimulation of transgenic islets in vitro with GH showed a reduced tyrosine phosphorylation of STAT-5 when compared with wild-type islets. Transduction of primary islet cultures with adenoviruses expressing various SOCS proteins followed by stimulation with GH or glucagon-like peptide-1 (GLP-1) revealed that SOCS-3 inhibited GH- but not GLP-1-mediated islet cell proliferation, indicating that the decreased β-cell volume observed in female transgenic mice could be caused by an inhibition of GH-induced β-cell proliferation by SOCS-3. In spite of the reduced β-cell volume the transgenic female mice exhibited enhanced glucose tolerance compared with wild-type littermates following an oral glucose-tolerance test. Together these data suggest that SOCS-3 modulates cytokine signaling in pancreatic β-cells and therefore potentially could be a candidate target for development of new treatment strategies for diabetes.