Glomeruli synthesize nitrite in active Heymann nephritis; the source is infiltrating macrophages. Glomeruli synthesize nitrite (NO₂^-) in experimental nephrotoxic nephritis, a model of glomerulonephritis where infiltrating macrophages are pathogenic. NO₂^- synthesis was studied in active Heymann nephritis (AHN), a model of membranous glomerulonephritis in which macrophages have not been implicated. Active Heymann nephritis (AHN) was induced with purified renal tubular epithelial antigen and adjuvants. Glomeruli isolated at seven to eight weeks after induction (proteinuria 183 ± 28 mg/24 hr, N = 6; adjuvant controls, 1.2 ± 0.8 mg/24 hr, N = 6) produced NO₂^- in culture spontaneously (7.1 ± 1.4, adjuvant controls 2.1 ± 0.9 nmol/2000 g/48 hours; N=0.021) and in increased amount following LPS stimulation (12.1 ± 2.8, controls 4.2 ± 1.6 nmol/2000 g/48 hours; P = 0.047). Synthesis was inhibited by L-NMMA, a competitive inhibitor of NO synthase. Enzymic digestion of glomeruli plus staining with mouse anti-rat macrophage monoclonal antibody EDI showed macrophage infiltration (32 ± 6, adjuvant controls 14 ± 2 macrophages/glomerulus; P = 0.002). Whole body irradiation (XR) suppressed NO₂^- production (LPS stimulated: 1.0 ± 0.4, N = 5; non-XR controls 7.2 ± 4.6 nmol/2000 g/48 hours; N = 5, P = 0.016) and macrophage infiltration (1.1 ± 0.5; non-XR controls 30 ± 12 macrophages/glomerulus; P = 0.008) but had no effect on proteinuria. Irradiation with renal shielding confirmed the close correlation between glomerular N0₂^- synthesis and glomerular macrophage numbers (r_s = 0.837, P < 0.001). These results show that macrophages infiltrate glomeruli in AHN; they are the source of NO₂^-in this model. Neither macrophages nor NO₂^- are the cause of proteinuria.