Twelve Large White pigs aged less than 1 min, and 3, 5.5, and 14 days were exposed to hypoxia (380 torr) for 2.5-3 days. The wall structure of terminal bronchiolar (resistance arteries) and elastic arteries was assessed by light and electron microscopy using quantitative morphometric techniques. In animals exposed from birth, mean terminal bronchiolar arterial medial thickness was increased (p less than 0.05) because endothelial and smooth muscle cells (SMCs) retained their fetal shape, position, overlap, interdigitation and the low surface/volume ratio characteristic of fetal life. In all older animals, the cells had a normal postnatal shape and surface/volume ratio. In the elastic vessels hypoxia did not prevent the normal postnatal reduction in mean SMC diameter of animals exposed from birth. SMC hypertrophy did not occur in any age group, but all animals save those first exposed at 14 days, showed an increase in SMC myofilament volume density (p less than 0.01). Connective tissue volume density also increased (p less than 0.01), mainly due to an increase in elastin and ground substance. Thus a short period of neonatal hypoxia impaired adaptation and appeared to potentiate contractile capacity in stiff-walled arteries but elicited a less marked response from animals first exposed at 14 days.