In vitro studies have suggested that leukotrienes are involved in acute allergic bronchoconstriction, though this has not been definitively corroborated yet in in vivo studies. On the other hand, edema production during antigenic challenge could be an additional factor favouring such bronchoconstriction. In the present work we quantified immunoreactive leukotriene C4 (iLTC4) concentrations in bronchoalveolar lavages during allergic bronchoconstriction induced by 1 mg/kg i.v. ovalbumin (OA) in immunized guinea pigs, as well as water content in guinea pig lung fragments obtained before and during this bronchoconstriction. We found that basal concentrations of iLTC4 (median 1.06 ng/ml) were not significantly modified at 2, 5 and 10 min (median 1.10, 0.29 and 1.37 ng/ml, respectively) of the bronchoconstrictor response. Water content in lung fragments did not change among non-immunized guinea pigs, immunized ones and at 15 min of bronchoconstriction (mean +/- SEM 79.32% +/- 0.18, 79.10% +/- 0.31 and 79.13% +/- 0.40%, respectively). In addition, isoproterenol (20 micrograms/kg, i.v.) rapidly reverted about 70% of the bronchoconstriction induced by a higher antigenic dose (OA, 3.1 mg/kg i.v.); residual obstruction was not associated with increased water content in lung fragments (78.13% +/- 0.43). These results suggest that in this model, acute allergic bronchoconstriction is not due to an increased iLTC4 release or to edema production, and that airway smooth muscle contraction is the main component of this response.