The effect of ethyl apovincaminate (RGH-4405, Cavinton) on the cerebral circulation has been studied with two methods in a series of neurosurgical patients. Regional circulation was studied with the H2 clearance method in five patients in whom deep electrodes were lodged in various cerebral structures with stereotactic surgery performed for the underlying disease. In connection with serial angiography of 25 patients 10 mg Cavinton was injected i.v.; circulation time of the contrast medium, arterial circulation time, changes of normal and pathological filling were appraised. Registered 15 min after administration, regional circulation showed significant increase, but slight increase was demonstrable in every structure investigated. The change was more marked in two elderly patients over 60 years. In eight cases of serial angiography marked difference was seen in filling by normal and pathological vessels on Cavinton effect; arterial circulation time changed in three cases, contrast medium circulation time did not in any of the cases. The most marked changes occurred in three cases of cerebral vascular disease. In two cases of glioma vascularization of the tumour was visualized by Cavinton.