Intrarenal blood flow and volume (indicator dilution technique), kidney volume (mercury in rubber resistance gage), intrarenal venous pressure, filtration fraction, and sodium excretion were determined sequentially before and during a 1 h infusion of isotonic saline 80 ml/kg in anesthetized dogs. The cortical fraction of renal blood flow rose during the first 20 min of infusion from an average of 70 to 77%, but returned nearly to control levels during the last 20 min of infusion because of a late rise in noncortical flow. During the first 20 min a 23% increase in cortical blood volume accounted for one third of the 8.5% increase in kidney volume, whereas in the last 20 min cortical blood volume had fallen nearly to control values and kidney volume was increased by 17.2%. Intrarenal resistances calculated from intrarenal pressure and flow indicated persistent cortical prevenous dilatation, progressive cortical venous constriction, and only a slight late reduction in noncortical resistance. These data indicate that hemodynamics are changing continuously during saline infusion and the natriuresis probably is multifactorial.