In order to study the effects of acute ureteral obstruction on glomerular hemodynamics, glomerular hydrostatic capillary pressure (PG), pressure in the first-order peritubular capillaries (EAP), and intratubular pressure (PT) were directly measured in superficial nephrons on Munich-Wistar rats by micropuncture with a servo-nulling device, in control conditions and one to two hours after ureteral ligation. Single nephron filtration fraction (SNFF) was calculated from arterial and peritubular blood protein concentration. SNGFR was measured by conventional micropuncture techniques in control conditions and was calculated from efferent arteriole blood flow (EABF) and SNFF during ureteral obstruction. EABF was obtained by timed complete collection of blood from superficial efferent arterioles. Afferent arteriole blood flow (AABF) and resistance of afferent (Ra) and efferent arterioles (Re) were calculated from conventional equations. Ureteral obstruction markedly increased PT from 12.9 +/- 1.4 to 36.8 +/- 6.1 (SD) mm Hg. The fall in SNGFR (from 23.3 +/- 6.4 to 17.9 +/- 5.2 [SD] nl/min) was blunted by the rise in PG (from 45.5 +/- 3.6 to 59.3 +/- 4.0 [SD] mm Hg) and AABF (from 130.0 +/- 59.1 to 144.2 +/- 69.0 [SD] nl/min), secondary to a fall in Ra. These results demonstrate that SNGFR is maintained early after complete ureteral obstruction because of afferent arteriole dilatation.