The effect of glucagon on the renal hemodynamics in the dog was examined by comparing its effect with that of secretin, a peptide with which glucagon shares a similar chemical structure. An intrarenal infusion of glucagon resulted in increases of RBF and GFR. GFR rose by approximately the same order of magnitude of RBF. The increase in GFR depended on the selective dilation of the afferent arteriole and a consequent rise in the transcapillary pressure difference. On the other hand, secretin infusion produced highly significant and proportional decreases in both afferent and efferent arteriolar resistance, resulting in no change in GFR. A superimposition of acetylcholine to glucagon decreased GFR even though RBF increased significantly. Glucagon infusion did not affect the permeability of glomerular capillary and the distribution of cortical blood flow. These findings indicate that the effect of glucagon on GFR depended on the selective dilation of afferent arteriole, and that as a result of its dilation the net filtration pressure increased without any change in permeability of glomerular capillary and a redistribution of filtration.