The reactions of cerebral pial vessels following the injection of normal saline(0.05cc, at 37 degrees C, pH7.4) and autogenous hemolytic blood(0.05cc, 0.1cc 0.2cc) into the cisterna magna of rabbits were tested in vivo. This experiment was performed in order to observe the effect of intravenous nimodipine(30 microgram/kg) injection at the maximally constricted time of the pial vessels, about 6 hours after intracisternal injection of autogenous hemolytic blood. The diameter changes of the pial vessels were measured under operating microscope through a cranial window on craniectomized area. Pial vessels were not significantly changed after an intracisternal injection of normal saline. But after intracisternal injection of autogenous hemolytic blood(0.05cc, 0.1cc, 0.2cc), pial vessels were maximally constricted at 6 hours and the constriction was continued during the experiments. Percentile constriction was increased as the amount of subarachnoid hemolytic blood increased. According to the results, pial vessels, especially small pial artery, were constricted by autogenous hemolytic blood containing oxyhemoglobin. Nimodipine, calcium antagonist, showed dilating effect on vasospasm after experimental subarachnoid hemorrhage with autogenous hemolytic blood in rabbits.