In 5 male (M) and 10 female (F) chickens the left pulmonary artery was surgically occluded (ligated) at 36 days of age (PAO). In a second group of chickens (7 M and 6 F) of similar age a similar surgical procedure was followed, but the left pulmonary artery was not occluded (SHAM). A third group of chickens (10 M and 6 F) was kept as controls (C). When the birds were 72 days old PO2, pH and haematocrit were measured in arterial blood, necropsies were performed and ventricular weights were obtained. SHAM birds showed no differences when compared to controls. All PAO birds had right ventricular hypertrophy (RVH) which was more marked in the males and 2 of the 5 PAO males had signs of right heart insufficiency. The ratio of the right ventricular weight to body weight was 0·60, 0·50, 1·37 and 0·82 (g per kg) for control M, control F, PAO M and PAO F respectively. The left ventricle of PAO birds was not affected. PaO2 in the PAO birds was lower than in the controls. Haematocrit was higher in the PAO M than in the controls, but it remained unchanged in the PAO F. These findings indicate that increasing the blood flow through one lung in chickens produces right ventricular hypertrophy and hypoxaemia, probably as a consequence of pulmonary arterial hypertension. Furthermore, the effects of left pulmonary artery occlusion in males seem to be more severe than in female chickens. © 1981.