 Immersion pulmonary edema IPE is a condition that has sudden onset in swimmers and divers, and is characterized by cough, shortness of breath, decreased blood oxygen levels, and hemoptysis. The purpose of this study is to examine the effects of cold water immersion and the dive reflex on pulmonary arterial pressure and pulmonary capillary wedge pressure in those who have already experienced IPE, as well as explore the possibility of a genetic predisposition. Healthy nonsmoking subjects who have experienced IPE will be recruited for several cold-water immersion experiments and DNA analysis. Their pulmonary arterial and pulmonary arterial wedge pressures will be measured as they undergo immersed rest and exercise trials in thermoneutral and cold water. If the pressures increase with these trials as hypothesized, the effects of sildenafil administration a pulmonary vasodilator will be tested during a second trial. Blood will also be drawn for DNA analysis of certain genes with possible relation to IPE. Results of these tests will be compared with those of the general population. 