Inflammatory mediators released in the airways during allergic reactions can affect the mucociliary system. Leukotrienes and platelet activating factor have been shown to affect the mucociliary activity in various cell preparations. The in vivo effects of these inflammatory mediators on mucociliary activity in rabbit maxillary sinus were investigated using a photoelectric technique. Leukotrienes C4 and D4 in the dose range 0.01-10.0 nmol/kg did not alter mucociliary activity significantly, either when given as bolus injections or as intraarterial infusions. Platelet activating factor had no effect in the dose range 0.001-0.1 nmol/kg. A dose of 1.0 nmol/kg increased mucociliary activity by 20.2 +/- 5.5% but produced adverse respiratory and circulatory effects at the same time. It is concluded that the inflammatory mediators leukotrienes C4, D4 and platelet activating factor have no essential regulatory influence on mucociliary activity in the upper airways of the rabbit.