Neural activities trigger regional vasodilation in the brain. Diffusible messengers such as nitric oxide (NO) and prostanoids are considered to work as vasodilators in neurovascular coupling. However, their roles are still controversial. In the present study, cortical images of neural activities and vasodilation were recorded through the intact skull of C57BL/6 mice anesthetized with urethane. Flavoprotein fluorescence responses elicited by vibratory hindpaw stimulation were followed by darkening of arteriole images reflecting vasodilation in the somatosensory cortex. Vasodilation was also observed in light reflection images at the wavelength of 570 nm in the same mice. We perfused the surface of the cortex under the skull with 100 mu M N-G -nitro-L-arginine (L-NA), an inhibitor of NO synthase (NOS), and 10 mu M indomethacin, an inhibitor of cyclooxygenase (COX). These drugs suppressed vasodilation without changing flavoprotein fluorescence responses. A mixture Of L-NA and indomethacin almost completely eliminated vasodilation. In mice lacking neuronal NOS (nNOS), activity-dependent vasodilation was significantly suppressed compared with that in littermate control mice, while that in mice lacking cytosolic phospholipase A2 alpha (cPLA2 alpha) was unchanged. These results indicate that NO works as a vasodilator in neurovascular coupling of the mouse somatosensory cortex. (C) 2007 Published by Elsevier Ireland Ltd and the Japan Neuroscience Society.