When passive Heymann nephritis (PHN) is induced by infusion of antibodies (anti-Fx1A), an acute fall in renal blood flow (RBF) and glomerular filtration rate (GFR) has been reported. Activation of the complement cascade by the local antigen-antibody reaction might be involved in this reaction. We therefore studied RBF and GFR during acute infusion of anti-Fx1A and after 3 days when heterologous antibodies are no longer present in the circulation. Two groups of rats were infused with 2 mg anti-Fx1A antibodies into the left renal artery; RBF was measured by the microsphere method and GFR by <sup>125</sup>I-Na-iothalamate clearance. In the first group, the measurements were made 40 min after the infusion, and in the second group after 3 days. A third group was studied 3 days after infusion of 1 mg anti-Fx1A. Animals infused with normal IgG were used as controls. Forty minutes after infusion of 2 mg anti-Fx1A, GFR in the left kidney was reduced from 1.16 ± 0.07 to 0.41 ± 0.21 vs. 0.94 ± 0.16 ml/min/g in the controls (p < 0.05). Three days after the infusion, GFR was 1.04 ± 0.07, not significantly different from control. RBF was reduced to 3.97 ± 1.11 ml/min/g after 40 min, compared to 7.53 ± 0.73 in controls (p < 0.05), and was normalized after 3 days. The effect of 1 and 2 mg anti-Fx1A antibodies was not significantly different after 3 days. Anti-Fx1A antibodies were detected in serum in the acute stage, but not after 3 days. The normalization of GFR and RBF pari passu with the disappearance of anti-Fx1A antibodies in serum indicate that the acute changes in renal hemodynamics may be due to complement activation during deposition of antibodies.