This study was performed to investigate the pulmonary vascular reactivity during hypothermia. It was found that the pulmonary vessels were more sensitive to alveolar hypoxia in hypothermic dogs, and that hypoxic pulmonary vasoconstriction (HPV) was enhanced, which could be inhibited by alpha 1-adrenoceptor blocker prazosin. The concentration of arterial plasma norepinephrine (NE) increased in dogs with hypothermia or in those with hypothermia plus hypoxia, but there was no significant difference between these two groups. The plasma level of 5-hydroxytryptamine (5-HT) was reduced during hypoxia in hypothermic dogs. Increase in plasma 6-keto-PGF1 alpha, which appeared in hypoxic dogs, was not observed during hypoxia in hypothermic dogs. These findings suggest that a potentiation in the sensitivity and reactivity of alpha-adrenoceptor in pulmonary vessels and a decrease in the modulation of PGI2 might be responsible for the enhancement of HPV. 5-HT seemed not to play a contributing role in the alteration of HPV.