The effect of hemodialysis on the cerebral circulation have been inadequately studied. Because of repeatability, safety and non-invasiveness transcranial Doppler sonography is well suited for evaluation of cerebral circulation and for estimation of cerebral blood flow during hemodialysis as well. The aim of our study was to find out whether hemodialysis affects the velocity of blood flow in extra- and intracranial arteries and in what way. 41 patients aged 18-71 (mean 43.9 +/- 13) years suffering from chronic renal failure made up the investigation group. We investigated 17 men and 24 women. Doppler examination was done to each dialized patient immediately before and then 120 and 240 minutes after the onset of hemodialysis. We used pulsed wave Doppler (TC2-64B, EME). Mean blood flow velocities were obtained from extracranial (CCA, ICA, ECA) and intracranial vessels (MCA, ACA, PCA). Simultaneously the series of biochemical blood parameters and arterial blood pressure were monitored. On the basis of investigation results the following conclusion have been drawn: Hemodialysis significantly affects on changes of blood flow velocities in extra-an intracranial arteries. Above mentioned changes consist in decrease of blood flow velocity in both CCA, left ICA, both MCA and PCA and right ACA.