Migration of L3 larvae of Nippostrongylus brasiliensis through the lungs of the rat, during primary infection, was studied at 24 h, 72 h and 8 days. At 24 h p.i., there was evidence of damage to lung epithelial cells and microvasculature, with increased protein and gamma-glutamyl transpeptidase in the bronchoalveolar lavage (BAL) fluid. However, there was little evidence of inflammatory cell recruitment. At 24 h p.i., there was a significant reduction in the inflammatory cytokine tumour necrosis factor alpha. Superoxide (O2-*) production was also reduced, accompanied by an increase in superoxide dismutase activity. Lipid peroxidation was reduced at 24 h p.i. and L3 larvae were shown to possess high levels of glutathione compared to host lung tissue. Nitric oxide, detected as nitrite, was produced in BAL fluid, and inducible nitric oxide synthase protein was increased by 72 h p.i. There was evidence of peroxynitrite production throughout the infection period with specific protein bands nitrosylated at 75, 30 and 25 kDa. It appears that despite early evidence of lung damage, the inflammation was reduced in response to L3 larvae of N. brasiliensis.