Collateral ventilation can participate in ventilation-perfusion regulation by shifting normoxic gas into hypoxic lung regions. In species lacking collateral pathways, such as cattle and swine, ventilation-perfusion balance must rely heavily on hypoxic vasoconstriction, which may explain why their muscular pulmonary arteries are much thicker than those of other animal species. The presence of these unusually muscular vessels in turn may account for the vigorous pressor response to acute hypoxia in these species. The only other species known to lack collateral ventilation is the coati. To determine whether coatis fit the pulmonary circulatory pattern of cattle and swine, we measured pulmonary arterial wall dimensions and pulmonary vascular responsiveness to acute airway hypoxia in 11 adult coatis. Hypoxia caused impressive pulmonary arterial hypertension [normoxia = 17 +/- 1 (SE) Torr, hypoxia = 40 +/- 2 Torr, cardiac output unchanged]. The medial thickness of muscular pulmonary arteries (50–300 microns) was 17.1 +/- 1.8% (SD) of external diameter, a thickness unprecedented in normotensive adult mammals. We conclude that coatis fit the pattern of other species lacking collateral ventilation, since they have thick-walled pulmonary arteries and a vigorous pressor response to hypoxia.