We explored the effects of GTN on the cerebral circulation with unprecedented temporal and spatial resolution by using two-photon laser scanning microscopy in anesthetized rats, where vessels were visualized by an i.v. injection of FITC dextran. Unexpectedly, we found that meningeal arterioles were abruptly constricted, while cortical arterioles were dilated. In contrast to the previous reports of high sensitivity of large veins to GNT, we found that small venules were nearly insensitive to this NO donor. In this study we, for the first time, employed two-photon laser scanning microscopy to visualize with high temporal and spatial resolution the acute effects of the GTN on meningeal versus cortical vessels. Using this technique, we found that cortical and dural vessels responded in opposing direction to the same treatment with the NO donor GTN. In the classical theory of Woolf, vasodilation was for long time considered as the leading reason for primary headaches like migraine. Nevertheless, recent data indicated that headache could be eliminated by substances which do not show any vasoconstrictory effects. Furthermore, substances like prostanoid PGF2a with strong vasoconstrictory effects do not produce headache in volunteers. Involvement of vasculature in migraine pain pathogenesis is still actively debated. Recent development of in vivo visualisation of blood vessels using multiphoton microscopy provides a powerful tool to address these issues with high resolution. Our present findings uncovered a previously unreported dichotomy in the reactions of intermediate diameter cortical versus dural vessels in the classical GTN model of headaches. These findings extend our knowledge about the complex behaviour of trigeminovascular system and, in particular, meningeal vessels innervated by trigeminal nerves where pain is likely generated. Tumor angiogenesis is a pathophysiological process involving the development of new capillaries and hyperpermeable blood vessels. For a tumor to grow beyond the occult stage, angiogenic activators have to outweigh inhibitors, leading to neovascularization. Activation of the angiogenic process, known as the “angiogenic switch”, is an important step in the progression from small lesions to malignant disease. Thus, it is important to be able to accurately monitor angiogenesis and its response to therapy. Many applications of quantitative DCEMRI to OTX015 detect response to anti-angiogenic and anti-vascular drug treatments assume that these methods are reproducible and can be used to predict biological changes. The inherent value of a biomarker is related to its biological variability relative to the reliability of measurement. Test-retest reproducibility of these biomarkers are important but have rarely been estimated. The current study evaluated the reproducibility of DCE-MRI with two contrast agents that have different molecular weights and hydrodynamic radii.