Mast cells contain heparin, histamine, hydrolytic enzymes, and possibly serotonin in metachromatic cytoplasmic granules, and are not visualized in routine histological preparations. Special fixation, frozen sections, and toluidine blue staining are essential for counting the number of mast cells in tissue sections. Histological preparations for counting mast cells were made from arteries of the circle of Willis in persons who died after chest or abdominal trauma (control group) and in patients who had subarachnoid hemorrhage (SAH) after aneurysm rupture. The arteries were removed within 6 hours of death, taking care to avoid damage to their structure, and were immersed in the fixative solution. This preliminary note, reporting findings in only a few cases, is justified by the interesting discovery of a marked increase in mast cell population in the muscular layer of arteries after SAH. The series is small because of the difficulty in obtaining suitable material, since mast cells virtually disappear when autopsy is performed later than 6 hours after death. It is concluded from this study that there is an increase of mast cell population in cerebral arterial walls after SAH, mainly in the muscular layer, and that the number of mast cells is higher in arteries closer to the aneurysm.