To determine if the glomerular filtration coefficient (K(f)) of the dog is influenced by changes in plasma colloid osmotic pressure (COP), we conducted micropuncture experiments in dogs given concentrated albumin solutions. In one group (N=9), filtration dynamics were evaluated following infusion of 450 to 600 ml of a 25% bovine albumin solution. To minimize the effects of acute volume expansion, we also achieved high COP levels in another group (N=7) by albumin loading on the day prior to the experiment. In all experiments, renal arterial pressure was reduced to approximately 90 mm Hg to minimize potential errors that might lead to overestimation of single nephron filtration rate (SNGFR) and glomerular pressure (GP). In the acutely expanded dogs. COP increased to 23.0 ± (SEM) 0.9 mm Hg. SNGFR was 59 ± 6 nl/min, estimated GP was 61.0 ± 2.0 mm Hg, proximal tubule pressure (PTP) was 23.0 ± 1.6 mm Hg, and superficial filtration fraction (SFF) was 0.13 ± 0.02. A similarly reduced whole kidney filtration fraction was also observed, due almost entirely to a marked increase in renal blood flow. When compared to noninfused control dogs (N=13), K(f) was significantly higher in the dogs with elevated COP, being 5.3 ± 0.6 nl/min/mm Hg as compared to 3.4 ± 0.3 nl/min/mm Hg. Average effective filtration pressure (EFP) was 12 ± 1 mm Hg, and EFP at the efferent end of the glomerular capillaries was 8.9 ± 1.2 mm Hg. In the group infused on the prior day. COP was 20.0 ± 0.8 mm Hg, SFF was 0.26 ± 0.01 SNGFR was 70 ± 8 nl/min, GP was 59 ± 2 mm Hg, and PTP was 19.0 ± 1.5 mm Hg. Average EFP was 15 ± 1 mm Hg, and EFP at the efferent end of the capillaries was 7.5 ± 0.7 mm Hg. K(f) was 4.85 ± 0.66 nl/min/mm Hg, a value significantly higher than that obtained in control dogs having a COP of 15.0 ± 0.6 mm Hg. Furthermore, one group of control dogs (N=4), expanded with an isooncotic albumin solution, did not exhibit significant changes in K(f) even through the degree of plasma volume expansion was similar to the group expanded with concentrated albumin solution. These experiments are consistent with previous findings obtained in the rat that K(f) is influenced by the COP, although the changes in K(f) appear to be less than they are in the rat. The data indicate that even under these conditions of elevated COP, the filtration process in the dog is characterized by positive filtration pressures throughout the length of the glomerular capillaries.