Thy 1 glomerulonephritis (GN) is a rat model of complement-dependent immune mesangial injury with induced glomerular nitric oxide (NO) synthesis. To examine mechanisms of inducible nitric oxide synthase (iNOS) induction, we studied the effects of treatment with the antioxidant N-acetyl-cysteine (NAC) and soluble complement receptor 1 (sCR1). Thy 1 GN was induced by intravenous anti-Thy 1 antibody. Glomeruli were isolated and kidney tissue taken from 30 min to 24 h after induction. Nitrite (NO<sup>–</sup><sub>2</sub>) synthesis, luminol chemiluminescence for reactive oxygen species (ROS), and iNOS and cytokine mRNA were assayed in isolated glomeruli. Mesangial injury (mesangiolysis) and leucocyte infiltration were quantitated on tissue sections. NAC (i.p. 1,000 mg/kg, 1 h prior to anti-Thy 1) reduced glomerular NO<sup>–</sup><sub>2</sub> synthesis (3.5 ± 0.66 vs. untreated 8.2 ± 1.1, p = 0.02), and iNOS mRNA expression, and abolished enhanced chemiluminescence. In vitro incubation of nephritic glomeruli with 20 mM NAC also suppressed nitrite production (4.7 ± 0.8 vs. untreated 12.2 ± 0.7 nmol NO<sup>–</sup><sub>2</sub>/2,000 glomeruli/48 h, p = 0.003), and chemiluminescence. In NAC-treated animals, neutrophil infiltration (0.5 ± 0 vs. untreated 9.6 ± 1.6 glomerulus, p = 0.0005), and macrophage infiltration (1.7 ± 0.4 vs. untreated 12.0 ± 0.1, p = 0.006) were abolished, and mesangiolysis was significantly reduced (45.9 ± 1.3 vs. untreated 34.4 ± 2.1 cells/glomerulus, p = 0.009). NAC did not inhibit anti-Thy 1 antibody deposition. C1q was unaffected, but C3 was reduced. sCR1 treatment prevented iNOS mRNA induction, the enhanced chemiluminescence, and the neutrophil infiltration at 1 h. IL-1β and TNFα mRNAs were not affected by either NAC or sCR1. These results show that NAC inhibits iNOS induction and NO synthesis in this model, and suppresses ROS synthesis and injury. They suggest that complement-dependent ROS generation is the critical initiating event that follows fixation of anti-Thy 1 antibody.