Micropuncture techniques were used to evaluate the determinants of glomerular filtration in hydropenic dogs. Stop-flow and servo-null techniques were used to estimate the glomerular capillary and proximal tubule hydrostatic pressures. The validity of stop-flow estimates was verified by comparisons with direct puncture of glomerular capillaries in Munich-Wistar rats. Efferent arteriolar oncotic pressure was calculated from the filtration fraction and systemic protein concentrations. This calculation was verified in separate experiments by measurement of the protein concentration in blood collected directly from efferent arterioles. In 14 dogs, estimated glomerular capillary pressure (GCP) averaged 65.8 +/- 2.9 mmHg and proximal tubule pressure (PT) averaged 20.5 +/- 1.3 mmHg. The net hydrostatic filtration pressure (GCP - PT) of 45.3 +/- 2.7 mmHg was significantly higher than the efferent arteriolar oncotic pressure (piEA) of 33.2 +/- 2.8 mmHg (P less than .001). These findings indicate that filtration dynamics in the dog are characterized by filtration pressure disequilibrium.