R ecent studies from this laboratory have revealed that single nephron filtration rate (sngfr) decreases significantly within 1 h of the administration of large doses of complement-fixing antiglomerular basement membrane antibody (AGBM Ab) in plasma-expanded Munich-Wistar rats. This reduction in sngfr was due to decreases in nephron plasma flow (rf) and the glomerular permeability coefficient (LpA) utilizing direct evaluation of all pertinent pressures, flows, and permeabilities. With identical micropuncture techniques, we have determined (a) the respective influences of rpf and LpA upon sngfr by examining the effects of differing doses of AGBM Ab, and (b) the specific effect of complement fixation upon the reduction in sngfr. In normal rats, low dose (1.4 microgram/g body wt) AGBM Ab decreased sngfr from 57.9 +/- 3.4 to 50.8+/- 3.9 nl/min per g kidney wt (kw) (P less than 0.001), and this was due to a 10% reduction in rpf and a decrease in LpA FROM 0.069 +/- 0.014 in control to 0.041 +/- 0.007 nl/s per g kw per mm Hg (P less than 0.02). At the high dose (2.3 microgram/g body wt), sngfr fell dramatically from 58.4 +/- 4.0 to 7.6 +/- 3.8 nl/min per g kw (P less than 0.001), and this effect upon filtration was the result of an 86% reduction in rpf and a decrease in LpA from 0.092 +/- 0.020 to 0.007 +/- 0.004 nl/s per g kw mm Hg (P less than 0.001). Therefore, at lower doses sngfr fell primarily as a result of a 40% reduction in LpA and a 10% decrease in rpf; however, at the high dose massive reductions in both rps and LpA led to the large decrease in sngfr. In complement-depleted rats, receiving identical doses, low-dose AGBM Ab no longer reduced the sngfr, but a reduction in LpA persisted (other factors compensating to maintain sngfr). At the high dose, complement depletion ameliorated the reduction in sngfr (55.1 +/- 2.4 to 37.2 +/- 3.4 nl/min per g kw mm Hg) by nearly eliminating the vasoconstriction but only partially diminished the reduction in LpA (0.097 +/- 0.020 to 0.032 +/- 0.004 nl/s per g kw mm Hg, P less than 0.05). Complement depletion prevented the migration of polymorphonuclear leukocytes (present in larger numbers after the high dose of AGBM Ab) into the capillary and eliminated vasoconstriction. Complement depletion resulted in a lesser effect of high-dose AGBM Ab upon LpA than in normal rats, and this is likely due to lesser polymorphonuclear leukocyte effects upon capillary surface area. The persistent reduction in LpA observed in complement-depleted rats correlated with separation of the endothelial cell from the glomerular basement membrane after AGBM Ab, AGBM Ab diminished glomerular ultrafiltration by decreasing LpA and altering the endothelial surface of the glomerular membrane, and this effect is not totally dependent upon the fixation of complement.