The objective of the present study was to determine the development of the smooth muscle of pulmonary arterioles through the relationship of the smooth muscle layer of pulmonary arteriole/arteriolar diameter (CM/DA). Ninety Cobb-Vantress chicks and 90 Ross chicks were used. Of these, 10 birds were weighed and slaughtered at 1 day of age, and the others were randomly distributed in a group raised at sea level (n=40) and a group raised at 3320 m above the sea level (n=40). Ten birds per group were weighed and slaughtered at 10, 20, 30 and 40 days of age. The CM/DA relation was calculated in lung slides stained with Mallory's phosphotungstic acid hematoxylin. Birds raised at sea level weighed more than those raised at high altitude (p<0.05), and chicks of the Ross line obtained higher body weight at sea level but lower at high altitude than those of the Cobb-Vantress line (p<0.05). The CM/DA relationship increased in all groups according to age (p<0.05), but the Cobb-Vantress line at sea level showed the higher CM/DA relationship from 20 days of age onwards (p<0.05).