The kinetics of radiolabeled heat-aggregated human IgG (AHIgG125I) were studied in rats with passive Heymann's nephritis (PHN) induced 72 hr previously with decomplemented rabbit antiserum to rat FX1A. Control rats were injected with decomplemented normal rabbit serum (NRS). Following administration of AHIgG125I (40 mg per 100 g of body wt) control and FX1A animals were sacrificed in groups of five each at 2, 4, 8, 16, and 24 hr and kidney, liver, spleen, lung, plasma, and blood cells obtained. 131I-Labeled human serum albumin (HSA131I) was administered prior to sacrifice as a plasma marker. In FX1A rats the following observations were made in comparison with control rats: (1) A decrease in the concentration of AHIgG125I in glomeruli was observed at 2, 4, and 8 hr after administration; (2) a significant increase in clearance reflected by a decrease in the concentration of plasma trichloroacetic acid (TCA)-precipitable radioactivity, and AHIgG125I (greater than 7 S) was present; (3) a significant increase in non-TCA-precipitable radioactivity in plasma and blood cells at most time periods; and (4) decreased concentrations of AHIgG125I in liver and spleen but not lung. The specificity of these observations was supported in separate experiments by the lack of any difference in the plasma levels of TCA-precipitable radioactivity after administration of radiolabeled albumin to FX1A and control rats. Studies in FX1A and control rats revealed no differences in body weight, kidney weight, hematocrit, blood volume, urine output, glomerular filtration rate, renal blood flow, or renal vascular resistance. A slight increase in urinary rat albumin excretion was observed in FX1A rats. The lower values of AHIgG125I observed in plasma, liver, and spleen associated with increased levels of non-TCA-precipitable radioactivity in plasma and blood cells suggest enhanced catabolism of AHIgG125I in FX1A rats, leading to decreased localization within the mesangium.