SUMMARY Peritubular capillary hydrostatic and oncotic forces and their relationship to the renal excretion of sodium (UNaV) were examined in 19 patients with moderate and uncomplicated essential hypertension (HT) and compared with data from 20 normotensire subjects (NT). Observations were made in hydropenia (C) and during sustained isotonlc saline volume expansion (E; 3% increase in body weight). The intrarenal renous pressure (IRVP) was used as an index of peritubular capillary hydrostatic pressure, and the efferent arteriolar colloid osmotic pressure (COP,n) was estimated from the arterial COP and the filtration fraction. C values (mean ± SEM) in HT (and NT) were: arterial pressure (MAP) 110 ± 3 mm Hg (85 ± 1, p< 0.001); glomerular filtration rate (GFR) 122 ± 4 ml/min/1.73 m* (128 ± 3, p> 0.05); renal blood flow (RBF) 1172 ± 38 ml/min/1.73 m1(1298 ± 48, p< 0.05); IRVP 25.0 ± 1.0 mm Hg (24.8 ± 0.8, p> 0.05); COP.tt33.0 ± 0.7 mm Hg (31.9 ± 0.6, p> 0.05); and UN.V 140 ± 13 $mole/min (161 ± 12, p> 0.05). During E, the increase of UH.V in HT was more than double that of NT (p< 0.001) while IRVP did not change in either group (p> 0.05) and COP.frfell by 26% (p< 0.001) in both groups. GFR and RBF increased by 18% (p< 0.001) and 19% (p< 0.001) respectively, in HT, but did not change in NT. MAP remained unchanged in both groups. The results indicate that the peritubular capillary physical factors are normal in established essential hypertension, and that these forces are not involved in the exaggerated natriuretic response to volume expansion in essential hypertension.