Aims: GPR56 is a member of the adhesion G-protein coupled receptor family that is known to regulate proliferation, apoptosis and organ development. Collagen III is its ligand in the developing brain, where its interaction with GPR56 is critical for the proper formation of the cerebral cortex. We have shown that GPR56 is the most abundant GPCR in human islets, but its function is not yet clear. Here, we investigated the role of GPR56 in insulin secretion, islet innervation and vascularisation, and whether it is activated by collagen III to influence islet functions. Methods: Post-natal day 9 (P9) wild type (WT) and GPR56 knock out (KO) mice were injected with BrdU (50mg/kg) intraperitoneally. Wax-embedded pancreas sections were immunoprobed for GPR56, collagen III, BrdU, Ki67, and the neuronal and vascular markers TUJ1 and CD31. Images were quantified by Image J. Insulin secretion was quantified by radioimmunoassay after WT and GPR56 KO islets were perifused with or without 100nM collagen III at 20mM glucose (20G). Changes in intracellular calcium were determined by microfluorimetry. Results: Islet GPR56 expression was confined to the beta cells while immunostaining revealed that collagen III is expressed by islet vascular endothelial cells. WT and GPR56 KO islets responded similarly to 20G (fold over basal; WT: 20.7±1.17, KO: 16.4±0.93, n=4). Collagen III potentiated insulin secretion from WT islets but not from KO islets (AUC WT, 20G: 37.4±3.0, +Col III: 55.0±5.4; KO, 20G: 33.5±5.6, +Col III: 36.2±4.1, n=4, p<0.05). Collagen III stimulated increases in calcium in both the presence and absence of extracellular calcium (basal to peak; +calcium, 20G: 0.02±0.005, +Col III: 0.04±0.005; -calcium, 20G: 0.003±0.002, +Col III: 0.006±0.002, n=24). The number of cells proliferating and still in the cell cycle was significantly lower in KO islets at P9 (BrdU+Ki67+ cells/µm2; WT: 115.9±18.2, KO: 50.9±6.3, n=3, p<0.05), but there were no differences in islet capillary density (number/µm2; WT: 0.03±0.007, KO: 0.02±0.009, n=3, p>0.2) or percentage islet nerve area (WT: 0.75±0.10, KO: 0.72±0.06, n=3, p>0.2). Summary: Our data suggest that GPR56 is activated by collagen III to regulate islet function and potentiate insulin secretion but it plays no role in islet innervation or vascularisation.