The effects of surgery and acute rejection on glomerular hemodynamics in the transplanted rat kidney are examined. Kidneys were transplanted from Munich-Wistar (MW) rats to syngeneic controls and MHC-incompatible PVG strain recipients. We report on 4 groups of animals: (1) two-kidney control MW rats; (2) unilaterally nephrectomized MW rats (UNX); (3) renal transplantation from MHC-identical MW littermates and removal of native kidneys (SYN); and (4) transplantation from MW donors to MHC-incompatible PVG rats and removal of native kidneys (ALLO). Glomerular filtration rate (GFR), single-nephron (SN)GFR, and glomerular capillary pressure (PGC) in SYN kidneys were depressed as compared to those in UNX (GFR, 1.41 +/- 0.08 vs. 0.80 +/- 0.08 ml/min; SNGFR, 67.2 +/- 4.8 vs. 44.7 +/- 6.6 nl/min; and PGC, 67 +/- 2 vs. 48 +/- 4 mmHg, UNX vs. SYN, respectively, P < 0.05). GFR and SNGFR in ALLO kidneys, however, were depressed even further (GFR, 0.40 +/- 0.05 ml/min; SNGFR, 13.8 +/- 1.8 nl/min; P < 0.05 for UNX vs. ALLO and SYN vs. ALLO). Afferent arteriolar resistance (RA) was increased greater than 4-fold (UNX, 1.01 +/- 0.15 10(10) dyn.sec.cm-5; SYN, 1.37 +/- 0.36 10(10) dyn.sec.cm-5; ALLO, 4.76 +/- 0.74 10(10) dyn.sec.cm-5 [P < 0.05, UNX vs. ALLO and SYN vs. ALLO]). This led to a precipitous fall in initial capillary flow rate in ALLO rats. These studies reveal the presence of moderate reductions in SNGFR and PGC in the nonrejecting transplanted kidney, which may relate to as yet unidentified consequences of the transplant surgery. More significantly, the principal mechanism leading to the reduced GFR characteristic of acute allograft rejection is identified as severe preglomerular vasoconstriction.