We studied the effects on ozone-induced airway hyperreactivity of U-60,257, a pyrroloprostacyclin shown to inhibit leukotriene C/D biosynthesis in vitro. A group of 5 guinea pigs were pretreated with U-60,257 (5 mg/kg IV), and studied before and 30 min after a 15 min exposure to 3.0 ppm ozone. These animals were compared to a similarly exposed group that was untreated (n = 10). Reactivity was determined by measuring specific airway resistance (SRaw) upon intravenous acetylcholine infusion in unanesthetized, spontaneously breathing animals. Prior to ozone exposure, we found that U-60,257 treatment did not affect either SRaw or muscarinic reactivity. After exposure to 3.0 ppm, all untreated guinea pigs showed substantial muscarinic hyperreactivity. In contrast, no significant change in SRaw or muscarinic reactivity occurred after ozone in any animal pretreated with U-60,257. We conclude that ozone-induced bronchial hyperreactivity in the guinea pig rapidly develops after a brief, high level exposure. This effect may be mediated, in part, by leukotrienes generated upon ozone exposure.