Fetal lung has vigorous hypoxic vasoconstriction and is exposed continuously to an O2 tension less than 20 mmHG, which is well below that experienced by adult lungs. We wondered if the newborn lamb, with muscular arteries similar to the fetus, might not have a different O2 tension for developing alveolar hypoxic vasoconstriction in addition to having stronger vasoconstriction than the less muscular adult lung. Therefore, regional hypoxic pulmonary vasoconstriction in newborn lambs (mean of 5 days) and adult sheep (older than 2 yr) was compared at graded levels of alveolar oxygen tension. Animals were anesthetized with pentobarbital, and each main bronchus was cannulated to allow ventilation of one lung with N2 or other O2/N2 mixtures as a "hypoxic" challenge while ventilation of the other lung with O2 maintained systemic oxygenation. Distribution of perfusion to the 2 lungs during bilateral O2 ventilation was first determined with 13N and a positron camera. The lambs significantly (p less than 0.05) decreased the distribution of perfusion to the test lung compared with O2 control (PAO2, 620 +/- 15) by 44 +/- 1% SEM with PAO2 of 22 +/- 1 mmHg, 22 +/- 3% with PAO2 of 96 +/- 1 mmHg, and 12 +/- 2% with PAO2 of 360 +/- 3 mmHg in contrast to the sheep that significantly decreased the distribution of perfusion to the test lung compared with O2 control distribution of perfusion by only 22% at a PAO2 of 19 +/- 2 mmHg and not at higher alveolar O2 tensions.(ABSTRACT TRUNCATED AT 250 WORDS)