Abstract--Glucagon, a major insulin counterregulatory hormone, binds to specific G_s protein-coupled receptors to activate glycogenolytic and gluconeogenic pathways, causing blood glucose levels to increase. Inappropriate increases in serum glucagon play a critical role in the development of insulin resistance and target organ damage in type 2 diabetes. We tested the hypotheses that: (1) glucagon induces proliferation of rat glomerular mesangial cells through glucagon receptor-activated phosphorylation of mitogen-activated protein kinase extracellular signal-regulated kinase 1/2 (p-ERK 1/2); and (2) this phosphorylation involves activation of cAMP-dependent protein kinase A (PKA) and phospholipase C (PLC)/[Ca²⁺]_i signaling pathways. In rat mesangial cells, glucagon (1 nM) stimulated [³H]-thymidine incorporation by 96% (P<0.01). This proliferative effect was blocked by the specific glucagon receptor antagonist [Des-His¹-Glu⁹] glucagon (1 µmol/L; P<0.01), a mitogen-activated protein kinase/ERK kinase inhibitor PD98059 (10 µmol/L; P<0.01), a PLC inhibitor U73122 (1 µmol/L; P<0.01), or a PKA inhibitor H-89 (1 µmol/L; P<0.01). The proliferation was associated with a 2-fold increase in p-ERK 1/2 that peaked 5 minutes after glucagon stimulation (P<0.01) and also was blocked by [Des-His¹-Glu⁹] glucagon. Total ERK 1/2 was not affected by glucagon. Pretreating of mesangial cells with U73122 or H89 significantly attenuated ERK 1/2 phosphorylation induced by glucagon. We believe that these are the first data showing that glucagon activates specific receptors to induce ERK 1/2 phosphorylation and thereby increase mesangial cell proliferation and that this effect of glucagon involves both PLC/[Ca²⁺]_i- and cAMP-dependent PKA-activated signaling cascades.