The objective of this study was to investigate the feasibility of continous monitoring of regional cerebral blood flow (rCBF) using a thermodiffusion probe in an animal model and in patients with severe head injury to show changes in rCBF during mild hyperventilation. An intraparenchymal ICP sensor, a multiparameter probe for determing the partial pressure of cerebral gases (pHti, ptiO2, ptiCO2) and a thermodiffusion probe for measuring regional cerebral blood flow were used in nine domestic pigs and in 10 intensiv care patients. To reproduce the changes in rCBF a hyperventilation maneuver was performed and the CO2-reactivity was calculated. The measurement of the partial pressure of oxygen (ptiO2) in the cerebral tissue served as a reference parameter. In the animal model as well as in the patient study hyperventilation produced a significant reduction in rCBF using the thermodiffusion probe. A good correlation between rCBF and the reference parameter ptiO2 was found in the animal model (R2 = 0,838). In the patient study, however, the correlation was weak (R2 = 0,38). In both studies a poor cerebral oxygenation was not always reflected by a low rCBF in some cases. The CO2-reactivity related to the rCBF was always detectabel in the animal model as well as in the patient study. The ptiO2-CO2-reactivity was partially restricted in the animal model. From a technical point of view, additional measurement of rCBF using thermodiffusion has proven to be a stable measuring method for clinical use. At this time, however, less experience is available than with regional ptiO2 measurement.