Cerebral blood flow (CBF) is high (mean of 50 ml/100 g. min) to cover metabolic and energy requirements of the brain. Independent from blood pressure in the systemic circulation CBF is kept constant by autoregulation, but it reacts to changes of arterial and tissue pCO2 and to metabolic needs of brain tissue resulting from functional activation. Below defined flow thresholds the function of the nervous tissue is abolished and its morphological integrity destroyed. Due to the regulatory mechanisms only a few drugs are able to affect CBF. The effects depend on the resting blood supply of small regions; this may lead to heterogeneous reaction patterns. The knowledge of drug effects on regional flow may be important in planning a treatment of cerebrovascular disease.