The effects of cyclosporin A (CsA), a potent immunosuppressive drug with antiparasitic activity, on the innate immunological response in guinea pig lungs during an early period (6th and 14th days) after T. spiralis infection were studied. CsA treatment of T. spiralis-infected guinea pigs caused a significant attenuation of immunological response in lungs by decreasing lymphocyte infiltration into pulmonary alveolar space, inhibiting alveolar macrophage superoxide anion production and lowering both the production of NO metabolites measured in bronchoalveolar lavage fluid and expression of the iNOS protein in lung homogenates, allowing us to speculate that the T. spiralis-dependent immunological response is dependent on lymphocyte T function. Interestingly, CsA itself had a pro-inflammatory effect, promoting leucocyte accumulation and macrophage superoxide production in guinea pig lungs. This observation may have a relevance to the situation in patients undergoing CsA therapy. Macrophage expression of the iNOS protein, evaluated by immunoblotting was not influenced by treatment of animals with CsA or anti-TGF-antibody, indicating different regulation of the guinea pig and murine enzymes.