Leukotrienes (LTs) have been implicated as mediators of the inflammation and ulceration associated with ulcerative colitis and Crohn's disease. In the present study, the effects of a novel, orally active inhibitor of LT synthesis (MK-886) were examined in a rat model of chronic colitis. Colitis was induced by intracolonic administration of trinitrobenzenesulfonic acid. Colonic LTB4 synthesis was measured after incubation of tissue samples in vitro and by in vivo equilibrium dialysis. A single dose of MK-886 (10 mg/kg) significantly inhibited colonic LTB4 synthesis for greater than 24 h. Daily treatment with this dose significantly reduced colonic damage, as assessed macroscopically and histologically, when the treatment was performed 2 h before induction of colitis and daily thereafter for 1 wk, but not when treatment was performed during the second week after induction of colitis. A less marked beneficial effect of MK-886 was observed when the pretreatment dose was excluded, suggesting a role for LTs in the early events of the inflammatory process. Inhibition of LT synthesis during the first 24 h after induction of colitis did not alter the extent of infiltration of neutrophils into the colon, as measured by tissue myeloperoxidase activity. Daily treatment with sulfasalazine (100 mg/kg po) either during the first or second week after induction of colitis did not significantly affect the rates of healing. At the dose used, sulfasalazine only produced a transient inhibition of colonic LTB4 synthesis. This study therefore demonstrates that a specific, orally active inhibitor of LT synthesis can significantly accelerate healing in this animal model of colitis when the treatment is performed during the early phase of the inflammatory response.