Free-flow and stop-flow intratubular pressures were measured in rats with an improved Gertz technique using Landis micropipets or a Kulite microtransducer. In hydropenia, average single nephron glomerular filtration rate was 29.3 nl/min, glomerular hydrostatic pressure (stop-flow pressure + plasma colloid osmotic pressure) was 70 cm H2O and mean glomerular effective filtration pressure was 12.7-14.3 cm H2O, approaching zero at the efferent end of the glomerulus. Thus, the glomerulus is extremely permeable, having a filtration coefficient four to five times greater than previously estimated. Mean effective filtration pressure and single nephron glomerular filtartion rate fell with elevated ureteral pressure and rose with volume expansion, more or less proportionately. Changes in effective filtration pressure were due primarily to increased intratubular pressure in ureteral obstruction and to reduced plasma colloid osmotic pressure in volume expansion; glomerular hydrostatic pressure remained constant in both conditions and thus played no role in regulation of filtration rate.