Previous studies have shown that the development of tolerance to nitroglycerin is related to a decrease in the release of endogenous calcitonin gene-related peptide (CGRP). In the present study, we explored whether endogenous CGRP is involved in reversal of tolerance to nitroglycerin with N-acetylcysteine or captopril in rats in vivo and vitro. Tolerance was induced by exposure to nitroglycerin (4.4×10 −6 M) for 10 min in vitro or by pretreatment with nitroglycerin (10 mg/kg, s.c.) three times a day for 8 days in vivo. Nitroglycerin (3×10 −9–10 −6 M) caused a concentration-dependent relaxation in the isolated rat thoracic aorta, an effect that was reduced by CGRP-(8–37) (3×10 −7 M) or capsaicin (3×10 −7 M). Preincubation with nitroglycerin for 10 min significantly decreased its vasodilation, which was restored in the presence of N-acetylcysteine (10 −5 M) or captopril (10 −5 M). Nitroglycerin (150 μg/kg, i.v.) produced a depressor effect and an increase in concentrations of nitric oxide and CGRP, and the effects of nitroglycerin disappeared after pretreatment with nitroglycerin for 8 days. However, tolerance to nitroglycerin in vivo also was partially restored in the presence of N-acetylcysteine or captopril. The present results suggest that reversal of tolerance to nitroglycerin with N-acetylcysteine or captopril is related to the increased release of CGRP in the rat.