Subarachnoid hemorrhage (SAH) results in a rapid decrease of cerebral perfusion. While cerebral perfusion pressure (CPP) may quickly recover, a sustained decrease of cerebral blood flow (CBF) has been observed. Acute vasospasm has been concluded from this mismatch. This study was conducted to visualize and investigate immediate vascular reactions during and after experimental SAH. Male Sprague-Dawley rats were subjected to SAH by the endovascular filament model (n = 7) or served as controls (n = 4). Videomicroscopy was performed via a cranial window. Regions of interest were defined in areas covered by videomicroscopy and arterial diameters measured at defined time-points from 15 min before until 3 h after SAH. Local CBF was monitored over the opposite hemisphere by laser-Doppler flowmetry. Local CBF showed a typical decrease immediately after vessel perforation followed by an incomplete recovery in the 3 h thereafter. Videomicroscopy demonstrated a sharp decrease of the arterial diameter in the first minutes after SAH. In some animals, SAH was followed by a complete disappearance of arterial vessel filling. In the following minutes, arterial filling reappeared or improved, respectively. All animals subjected to SAH showed significant vasospasm in subarachnoid arteries. This is the first study to visualize acute vascular reactions during and immediately after SAH. Although the cranial window technique only covers a part of the cerebral vasculature, it covers cerebral vessels rather distant from the site of endovascular perforation. Therefore, it is likely that acute vasospasm observed in the monitored areas reflects a global vascular reaction.