Dysfunction and destruction of pancreatic islet β-cells is a hallmark of diabetes. Better understanding of cell signals regulating β-cell growth and antiapoptosis will allow development of therapeutic strategies for diabetes by preservation and expansion of β-cell mass. GH and IGF-I share a complicated physiological relationship and have both been implicated in β-cell function. GH and IGF-I exert their biological effects through binding to respective receptors (GHR and IGF-IR) and subsequently engaging downstream signaling pathways. However, their collaborative roles in modulation of β-cell mass and the underlying molecular mechanisms remain poorly understood. In this study, we demonstrate that cultured β-cells are appealing systems for investigating potential GH-IGF-I signaling cross talk. We uncover that GH specifically promotes formation of a protein complex containing GHR, Janus kinase 2 (a nonreceptor kinase coupled to GH/GHR signaling), and IGF-IR. More importantly, GH and IGF-I synergistically activate both signal transducer and activator of transcription 5 and Akt pathways. Concomitantly, β-cells proliferate more robustly and are better protected from serum deprivation-induced apoptosis when exposed to GH and IGF-I in combination vs. GH or IGF-I alone. The augmented proliferative effects by GH and IGF-I are confirmed in isolated islets. Taken together, our findings strongly suggest that there exists a novel signaling relationship between GH/GHR and IGF-I/IGF-IR systems in β-cells, i.e. IGF-IR may serve as a proximal component of GH/GHR signaling, contributing to enhancement of β-cell mass and function. In support of this, IGF-IR knockdown in β-cells resulted in the desensitization of acute GH-induced signal transducer and activator of transcription 5 activation. © 2011 by The Endocrine Society.