A quantitative light microscopical and ultrastructural study of the elastic and large muscular pulmonary arteries of 30 Large White pigs aged from birth to 6 months yielded light microscopical measurements on 120 arteries and 62 560 ultrastructural assessments, which composed a computerised database. After birth mean arterial medial thickness and mean smooth muscle cell diameter decreased during the first 4 days (p<0.01). Mean interlamellar distance decreased, reaching its nadir at 1–3 weeks (p<0.01). All these features increased between 3 weeks and adulthood (p<0.01). In the smooth muscle cells synthetic rather than contractile organelles were dominant during the first 3 weeks. Between 3 weeks and adulthood, however, smooth muscle cell myofilament volume density increased (p<0.0001). At all ages the smooth muscle cells of the outer lamellar units of elastic arteries had a greater myofilament volume density than those of adluminal units (p<0.0001). The amount of connective tissue increased between 3 weeks and adulthood, collagen and basement membrane increasing preferentially (p<0.0001, p<0.05, respectively). Thus the conducting pulmonary arteries, like the peripheral resistance arteries, adapt structurally to extrauterine life. Remodelling occurred rapidly after birth, and then gradually during growth, as connective tissue was deposited and smooth muscle cells matured.