The influence of age on endothelial functional markers was investigated in rats. Angiotensin I converting enzyme (ACE) activity and nitric oxide synthase (NOS) mRNA expressions were examined in the lung and aorta of 10-, 20-, and 30-mo-old normotensive rats. These data were extended by the measurement of circulating endothelial cells. ACE activity was significantly decreased in plasma (P < 0.01) and lungs (P < 0.01) at 30 mo, whereas it was significantly increased in the aorta (P < 0.001) at this age. Conversely, ACE mRNA levels decreased with age in the lung (P < 0.05). The level of constitutive endothelial NOS (eNOS) mRNA was significantly reduced in the aorta of 30-mo-old rats (P < 0.05), but no changes were observed in the lungs. The level of inducible NOS (iNOS) mRNA in the aorta was significantly decreased in 20- and 30-mo-old rats (P < 0.01), whereas it was significantly increased in the lung at 30 mo (P < 0.01). Interestingly, eNOS was expressed approximately 30 times more (P < 0.001) in the aorta than iNOS, whereas in the lung it was only slightly higher than iNOS (35%; P < 0.001). Neuronal NOS mRNA expression was not modified with aging. In the aorta, guanosine 3',5'-cyclic monophosphate concentration followed NOS expressions and showed a significant decrease at 30 mo (P < 0.001). An increase in the number of circulating endothelial cells was observed in the oldest rats, possibly reflecting an increase in endothelial cell turnover with aging. The present results demonstrate that aging modifies the expression of endothelial markers implicated in the regulation of vasomotor tone. This age-dependent impairment of endothelial functions could contribute to the increased risk of pathological processes within the arterial wall associated with aging.