The physiological basis for the increase in pulmonary arterial pressure induced by cold exposure of cattle will be examined. It has been shown that cold exposure causes a relative hypoventilation leading to a fall in arterial oxygen tension and a rise in carbon dioxide tension. Thus part of the increment in pressure may be due to blood gas changes. This will be tested by allowing the animals to breathe an oxygen enriched atmosphere under cold and thermoneutral conditions. In order to examine the cause of the cold-induced hypoventilation hypoxic sensitivity under cold exposure will be tested. The role of deep core and peripheral thermoreceptors in pulmonary hypertension will be determined from measurements of mean skin temperature and deep body temperature. The interaction between cold-induced pulmonary hypertension and that resulting from hypoxia will be examined by exposing animals to various altitudes under both cold and thermoneutral conditions.