The effect ofMontelukast sodium, a leukotrien blocker, on mean pulmonary arterial pressure was evaluated in chickens with ascitis syndrome. A total of 60 mal e broilers of the Arbor Acres line, bom and reared at sea level, were studied. At 30 days of age, they were divided in two groups, one control and one experimental, of30 birds each. The latter group received Montelukast sodium orally (0.2 mg/kg body weight) daily for 5 consecutive days. At 35 days, mean pulmonary arterial pressure (mPAP), hematocrit (Ht) and body weight (BW) were recorded for both groups. Direct catherization ofthe left pulmonary artery with a pressure transducer connected to a recording system described by Burton was used to detennine mPAP. Average values were 21.89 ± 5.61 forthe control group and 18.57 ± 4.89 mmHg for the experimental group. Hematocrits were 31.33 ± 3 .89 for the control group and 32.50 ± 2.22 % for the experimental group, while body weight values were 1667 ± 233 and 1769 ± 184 grespectively.Although Montelukast sodium treatment significantly decreased mPAP (P<0.05), a similar effect was not observed for Ht or BW (P>0.05). It is suggested that the decrease in mPAP was dueto vasodilatation produced by the blocking effect of Montelukast sodium on vasoactive leukotrien receptors.