The effect of increased plasma oncotic pressure on renal blood flow (RBF), glomerular filtration rate (GFR), electrolyte excretion, and renin secretion rate (RSR) was studied in dogs anesthetized with sodium pentobarbital. Renal artery infusions of hyperoncotic dextran or human serum albumin raised renal venous colloid osmotic pressure an average of 7.3 and 10.1 mmHg, respectively, and caused small but consistent increases in RBF, large increases in RSR, marked decreases in urine flow rate and electrolyte excretion, with either no change or small decreases in GFR, and no change in renal artery pressure. Renal vasodilation was confined primarily to afferent arterioles and was not measureable until approximately 45 s after the start of infusions. The renal responses to increased plasma oncotic pressure appeared to be an autoregulatory phenomenon, consistent with a tubular mechanism dependent on an altered distal tubular fluid flow and/or composition. The increased renin release during increased plasma oncotic pressure is not compatible with a renal baroreceptor mechanism that responds to decreases in afferent arteriolar pressure because calculated glomerular pressure increased during albumin and dextran infusions.