The objective of the present investigation was to examine the effects of an inhaled glucocorticoid, budesonide, on antigen-induced production of cysteinyl leukotrienes (cys-LTs) and pulmonary inflammatory cell infiltration in the Brown Norway rat, an animal model of asthma. Two weeks after sensitization to ovalbumin, rats were treated with budesonide (2.5 mg/kg) 18 and 1 h before challenge with antigen. Budesonide abolished the late response to ovalbumin (P < 0.02) and strongly inhibited the in vivo synthesis of N-acetyl-leukotriene E₄, an indicator of cys-LT synthesis, during this period (P < 0.005). Both total bronchoalveolar lavage (BAL) cells (P < 0.01) and BAL macrophages (P < 0.005) were markedly reduced to ~25% of their control levels after treatment with budesonide. It can be concluded that inhibition of the antigen-induced late response in Brown Norway rats by budesonide is associated with reductions in both BAL macrophages and cys-LT synthesis. It is possible that the effect of budesonide on cys-LT synthesis is related to its effects on pulmonary macrophages.