Background This study was designed to demonstrate the development of biochemical tolerance to organic nitrates by measuring levels of exhaled gaseous nitric oxide (NO) in lambs given intravenous (IV) nitroglycerin or sodium nitroprusside.Methods and Results IV injections of nitroglycerin or sodium nitroprusside produced dose-dependent and sustained increases in the exhaled levels of nitric oxide measured by chemiluminescence in awake lambs with tracheostomies. After a 6-hour IV infusion of 25 micrograms x kg sup -1 x min sup -1 nitroglycerin, peak exhaled NO levels were significantly reduced (-53.6+-4.9%, mean+-SEM, P<.001) and systemic hypotensive responses were attenuated (-52.6+-5.9%, P<.001) after an IV challenge of nitroglycerin but not sodium nitroprusside. After a subsequent 12-hour nitroglycerin-free period, there was complete recovery of NO excretion in exhaled breath and a return to baseline of systemic hypotensive changes on administration of IV nitroglycerin boluses. For IV sodium nitroprusside challenges, pulmonary NO excretion and systemic hypotensive responses remained constant throughout the study. Challenges with IV nitroglycerin but not sodium nitroprusside during a 12-hour nitroglycerin-free period resulted in delayed biochemical recovery with various exhaled NO levels and systemic hypotensive responses to challenges with IV nitroglycerin.Conclusions Measurements of exhaled NO provide in vivo, noninvasive evidence for the development of biochemical tolerance to nitroglycerin. There was reduced NO release into exhaled gas from the pulmonary vasculature concomitant with evidence of tolerance to nitroglycerin vasodilation in the systemic circulation. (Circulation. 1994;89:2498-2502.)