A revision is made of the methods employed up to date in determining cerebral blood-flow by means of radioactive isotopes. The quantitative methods are compared to the qualitative ones regarding their clinical and diagnostic value. While quantitative methods allow an exact determination of the cerebral blood-volume, and thus of the blood-flow through the entire brain or through certain of its areas, qualitative methods offer special advantages because of their clinical and diagnostic value. By determining cerebral circulatory time and tracing the activity curves enregistered over both hemisphere it is possible to obtain differential diagnostic conclusions on the presence of a tumor, an arteriovenous malformation, as well as on an arteriosclerotic cerebral process. The test, besides taking only a few minutes, does not overload the patient neither does it expose him to a significant amount of radiation. It can also be performed on older patients, to whom angiography is not entirely free of risks.