Abstract We studied the filtration characteristics of glomeruli isolated from the superficial and deep renal cortex of Munich-Wistar rats (180 to 300 gm) to determine whether the Ultrafiltration coefficient (K f) or hydraulic conductivity (Lp) of the glomerular capillary Is altered during expansion of extracellular fluid volume with isotonic protein-free (SAL) or hyperoncotic (ALB) solutions. SAL rats were given an initial infusion of 1% dextrose in lactated Ringer's solution (0.1 ml/gm body weight) followed by a maintenance infusion for 18 to 20 hours. ALB rats were given hyperoncotic bovine serum albumin (0.01 gm/gm body weight) intravenously 2 hours before sacrifice. At sacrifice, urine flow rate and fractional excretion of sodium were increased and plasma renin activity decreased in both SAL and ALB rats. Expansion of intravascular volume was evidenced by a decrease in the hematocrit of ALB but not of SAL rats; plasma protein concentration was increased in ALB and unchanged in SAL rats. K f and Lp were estimated during in vitro filtration. Because SAL rats were significantly larger than ALB rats, results were compared with those in separate size-matched controls. K f of superficial glomeruli of large control and SAL rats did not differ (3.7 ± 0.4 and 3.5 ± 0.4 nl/min/mm Hg, respectively). Lp, of the same glomeruli, calculated as the quotient of K f and estimated filtering area (Lp d), were also comparable (1.9 ± 0.2 and 1.8 ± 0.2 μl/min/mm Hg/cm 2). K f of superficial glomeruli of ALB rats was significantly greater than K f of size-matched control rats (3.7 ± 0.3 vs. 2.9 ± 0.1 nl/min/mm Hg, P < 0.05). Lp D was also elevated in superficial glomeruli of ALB rats (2.5 ± 0.2 vs. 2.0 ± 0.1 μl/min/mm Hg/cm 2, control values). K f and Lp D of deep glomeruli of large and small control, SAL, and ALB rats did not differ (average K f 4.0 ± 0.3, 4.2 ± 0.1, 3.8 ± 0.4, and 4.4 ± 0.8 nl/min/mm Hg and Lp D 2.1 ± 0.1, 2.2 ± 0.1, 1.8 ± 0.1, and 2.2 ± 0.3 μl/min/mm Hg/cm 2). We conclude that expansion of intravascular volume with hyperoncotic solution increases K f by increasing the capillary hydraulic conductivity of superficial but not of deep glomeruli. This effect is not a direct result of increased plasma oncotic pressure inasmuch as it does not occur in deep glomeruli exposed to plasma of the same composition as that perfusing superficial glomeruli. Increased K f and Lp do not occur during infusion of isotonic protein-free solution despite suppression of plasma renin activity.