In anesthetized mongreal dogs, the intrarenal arterial (0.2 approximately 1.0 unit/kg-min) and the intravenous infusion (0.4 approximately 2.0 unit/kg-min) of secretin caused dose-dependent increase of RBF, accompanied by decreases of the calculated afferent arteriolar resistance (Ra) and efferent arteriolar resistance (Re), but produced no significant effect on GFR, urine flow, electrolyte excretion, osmolar clearance and free water reabsorption. The distribution of cortical blood flow was examined using the radioactive microsphere technique. The intrarenal infusion of secretin (1.0 unit/kg-min) increased renal cortical blood flow in the juxtamedullary area much more than in the superficial area, shifting the blood flow from the outer to the inner zone. Simultaneous intrarenal infusion of secretin (1.0 unit/kg-min) and glucagon (0.5 microng/kg-min) produced increases in GFR, urine flow and electrolyte excretion to a lesser degree than those induced by glucagon alone, whereas the increment in RBF and the decreases in Ra and Re were almost to the same degree as those caused by secretin alone. The present results indicate that secretin produces the dilation of afferent and efferent arterioles, resulting in an increase in RBF, with no change in GFR and urine flow, and that the effects of glucagon on renal functions are masked by secretin mainly through the effects of renal hemodynamics.