Fifteen healthy current smokers with a >10 pack-year history of smoking and 15 healthy never-smokers will be enrolled. Males and females between 25 and 75 years of age will be recruited There will be 1-4 visits to the laboratory (one for non-smokers, 4 for smokers). All subjects will have the first visit. Current smokers will have 3 additional visits. Current smokers will be asked not to smoke before coming to the laboratory on the 3 study days. All subjects will be instructed to abstain from ingesting alcoholic beverages, coffee or caffeinated drinks for at least 12 hours the night before each study day. The subjects will also be instructed not to use PDE5 inhibitors for 24 hours before coming to the laboratory. On each study day, the protocol will start at the same time (in the morning).