The static elastic properties and structure of the intra-pulmonary arteries have been studied in pigs from fetal life to 4 months old, by radiographic and histological methods. During this period the average radius of the vessels examined varied from ≈0.9 mm at birth to 3.3 mm at the age of 4 months. The form of the pressure radius curves was similar to that observed in the systemic circulation and was characteristic of the gradual recruitment of collagen fibres with increasing pressure. In vessels near the hilum, wall thickness decreased from ≈180 μm to 100 μm during the first 2 weeks and thereafter increased to ≈250 μm. Throughout the lung and in all age groups there was a linear relationship between the logarithm of pressure strain elastic modulus (used as a measure of functional stiffness) and pressure. In contrast to the systemic arteries there was little change in functional stiffness towards the periphery, although the incremental elastic modulus (structural stiffness) did increase at sites further removed from the heart. During the first two weeks of life there was little change in functional stiffness (Ep ≈ 1.1 × 104 Nm2) although by 4 months Ep had increased by a factor of three. During this period despite fluctuations with age, an overall increase in structural stiffness was also observed (EINC ≈1 × 105 at 4 to 6 h to 2 × 105 Nm−2 at 4 months). The observations suggest that the changes in elasticity observed in early life are a consequence of the adaptation of the circulation to the lower pulmonary arterial pressure of extra-uterine life.