The effects of mock CSF, nilvadipine solution of various concentrations and PEG 400 solution on regional cerebral pial vessels in rabbits were studied by topical microapplication to the perivascular evironment through the bilateral cranial windows in a randomized fashion. Physiological parameters(PaO2, PaCO2, blood pH, and systolic blood pressure) were not significantly changed during all experiments. The pial vascular diameter was directly determined with the micrometer eyepiece under the operating microscope. The results were the followings: 1) Topical applications with mock cerebrospinal fluid(Group I) and PEG 400(Group II) on cerebral pial vessels did not significantly change pial vascular diameter in comparison with the resting state(p>0.05). 2) Application of nilvadipine solution of 1x10(-8)M did not show significant dilatation, and solutions over the range of 1x10(-7) to 1x10(-2)M resulted in significant dilatation of the cerebral pial arteries in a dose-dependent manner. The small arterial segments showed more dilatation than large arterial segments when topical nilvadipine solution were applied, however, the difference was not significant(p>0.05). 3) In venous segments, topical application of the nilvadipine solution induced no significant pial vein dilatation compared with the resting state(p>0.05), except when 10 minutes after the topical application of 1x10(-2)M nilvadipine solution(p<0.05). It may be suggested that topical application of nilvadipine solution induce the dilatation of pial arteries with dose-dependent manner. Nilvadipine might be used for treatment and prevention of cerebral vasospasm and ischemia.