Two-hundred one-day-old male (M) and female (F) chickens were exposed to 3300m (HA). Two-hundred control chickens were raised at sea level (SL). Chickens from both HA and SL were studied each week from the 3rd to the 7th week of age. Pulmonary arterial pressure (Ppa) was measured under local anesthesia in conscious animals. Hb, Hct and red blood cell counts (E) and the weights of the right ventricle (RV), left ventricle (LV) and septum (S) were obtained. HA chickens, both males and females, reached lower body weights than SL chickens. Ppa were 16 and 43 mm Hg (2.13 and 5.73 kPa) while RV/(RV+LV+S) values were 0.195 and 0.347 in SL and HA chickens, respectively. RV was 90% greater and LV 20% smaller in the HA chickens. S was not affected by altitude. Hb, Hct and E were higher in the HA birds. Sixteen HA M and 1 HA F died with signs of right heart insufficiency (RHI), however, M did not show greater values of Ppa or RV/(RV+LV+S) than F at SL or at HA. This indicates that the higher incidence of RHI in the M cannot be attributed to a higher responsiveness of their pulmonary vasculature to hypoxia. © 1980 Springer-Verlag.