Peripheral pulmonary arteries remodel immediately after birth as pulmonary vascular resistance falls. We hypothesised that there is a link between the response to agonist stimulation and wall structure in early postnatal life. Arteries from fetal, newborn and 14-day-old piglets were mounted on a perfusion myograph. Lumen diameter and smooth muscle cell nuclear positions were recorded after sequential addition of KCl, U46619 and bradykinin. Subsequently, vessels were studied by confocal and transmission electron microscopy. Contractile agonist stimulation caused a greater and faster reduction in lumen diameter in the newborn than at 14 days (p < 0.02) and the distance between cells decreased more in the newborn (p < 0.05). Unstimulated smooth muscle cells changed shape from being rounded in the fetus to fusiform by 14 days, with increasing length/width ratio (p < 0.01). On contraction, length/width ratio decreased most at 14 days (p < 0.01). Extracellular matrix, collagen and elastin, sparse at birth, increased by 14 days. Thus a greater change in lumen diameter in relation to cellular contraction was observed in the newborn than at 14 days and the amount of extracellular matrix within the vessel wall may affect the overall change in lumen diameter.