Intra-aortic measurement of nitric oxide (NO) would provide valuable insights into NO bioavailability in systemic circulation and vascular endothelial function. In the present study, we thus developed a catheter-type NO sensor to measure intra-aortic NO concentration in vivo. An NO sensor was encased and fixed in a 4-Fr catheter. The sensor was then located in the thoracic aorta via the femoral artery through a 7-Fr catheter to measure intra-aortic plasma NO concentration in vivo in anesthetized dogs. Infusion of acetylcholine (10 mug/kg) increased base-to-peak plasma NO level in the aorta by 2.4 +/- 0.4 nM (n = 7). After 20-min infusion of N-G-methyl-L-arginine (NO synthase inhibitor), changes in plasma NO concentration in response to acetylcholine were attenuated significantly (1.8 +/- 10.4 nM, P < 0.003, n = 7). In conclusion, the newly developed catheter-type NO sensor successfully measured acetylcholine-induced changes in intra-aortic plasma concentration of endothelium-derived NO in vivo and demonstrated applicability to direct evaluation of intravascular NO bioavailability. (C) 2003 Elsevier Science (USA). All rights reserved.