Micropuncture experiments were conducted on 12 dogs to allow calculations of the filtration coefficient (Kf) and the effective filtration pressure (EFP) of the glomerular capillaries. Glomerular pressure (GP) was estimated from the sum of the stop-flow pressure and plasma colloid osmotic pressure, and single nephron filtration rate (SNGFR) was measured by total collections from proximal tubules. Since previous studies indicate that at normal arterial pressures the interpostition of an oil block in the proximal tubule and the consequent interruption of distal tubular flow may lead to an overestimation of GP and SNGFR, renal arterial pressure (RAP) was reduced to the lower level of the autoregulatory range. At the reduced RAP of 87±6 mm Hg, renal blood flow and glomerular filtration rate (GFR) were 3.6±0.6 (SD) and 0.68±0.13 ml/min . g, values which were not significantly different from control. Whole kidney filtration fraction averaged 0.35±0.04 and was not statistically different from the filtration fraction calculated in eight experiments utilizing measurements of efferent arteriolar blood hematocrit (0.35±0.09). Proximal tubule pressure was 20±2.4 mm Hg, and peritubular capillary pressure averaged 12.2±3.3 mm Hg. At reduced RAP, stop-flow pressure averaged 38.2±3.1 mm Hg and colloid osmotic pressure averaged 14.2±1.7 mm Hg, yielding a value of 52.3±3.7 mm Hg for GP. SNGFR was 55.9±9.4 nl/min. Calculated values for Kf based on whole kidney and superficical nephron filtration fractions were 4.7±1.1 and 5.0±1.2 nl . min-1 . mm Hg-1, respectively. The average effective filtration pressure (EFP) values were 12.2±2.2 amd 11.5±2.6 mm Hg. Estimated EFP at the terminal end of the glomerular capillaries was significantly greater than zero, indicating that the filtration process occurs throughout the length of the glomerular capillaries in superficial nephrons of the dog.