The effect of four Toxocara canis antigens on nitric oxide (NO) and prostaglandin E2 (PGE2) synthesis was studied in vitro using rat alveolar macrophages. Somatic and excretory/secretory T. canis antigens prepared from adult worms and LII larvae were incubated with rat alveolar macrophages obtained by bronchoalveolar lavage at concentrations of 0.1-50 microg/ml. Both excretory/secretory adult antigen (ESA) and somatic LII antigen (SLII) stimulate the release of nitrites by alveolar macrophages. This effect was specific (inhibited by L-NAME and L-canavanine) and dose-dependent; 30 microg and 10 microg being the most effective concentrations of ESA and SLII, respectively. Western blot and reverse transcriptase-polymerase chain reaction analyses revealed that ESA antigen stimulates the production of NO at transcriptional level. T. canis ESA also stimulated macrophages to produce PGE2 at transcriptional level. The addition of L-canavanine decreased the release of PGE2 significantly, which suggests that NO mediates the production of this prostaglandin. These results indicate that T. canis can stimulate the release of vasodilatory mediators by macrophages of the host.