Allergen challenge of sensitized Brown-Norway (BN) rats results in increased excretion of cysteinyl-leukotrienes (cLTs) in bile. It is unclear whether this reflects an increased capacity of lung cells to synthesize 5-lipoxygenase products, and, if so, which cells are of primary importance. We have examined the effects of allergen challenge on the capacity of a mixture of isolated lung cells from ovalbumin (OA)-sensitized BN rats to synthesize LTs and other eicosanoids. Cells were isolated by enzymatic digestion of lung tissue before and either 6 or 24 h after challenge of sensitized rats with either OA or saline. A23187-induced synthesis of eicosanoids by these cells was measured using high-pressure liquid chromatography. OA challenge resulted in a significant influx of neutrophils into the lungs and a significant increase in the synthesis of 5-lipoxygenase products, in particular LTB4, by lung cells after 6 h. There was a positive correlation between the percentage of neutrophils in unfractionated lung cells and the amounts of LTB4 produced by these cells. OA challenge had little or no effect on the production of cLTs and the cyclooxygenase product 12-hydroxy-5,8,10-heptadecatrienoic acid. There was a significant increase in the infiltration of eosinophils into the lungs 24 h after OA challenge but no increase in the production of cLTs by lung cells at this time, suggesting that eosinophils from BN rats are unlikely to be the major site for the production of these substances. This was confirmed in experiments with partially purified eosinophils obtained from Sephadex-treated rats. In contrast, cLTs were major products of arachidonic acid metabolism by alveolar macrophages from BN rats. We conclude that allergen challenge results in an increased capacity of lung cells to synthesize 5-lipoxygenase products, in particular LTB4. Macrophages, rather than eosinophils, may be an important site for the synthesis of cLTs in BN rat lungs.