The authors' aim was to evaluate the response of the intracranial venous system to application of functional tests. In 46 healthy volunteers, the basal veins were examined using venous transcranial color-coded duplex sonography. We evaluated angle-corrected venous peak-systolic flow velocities (V(vb)) in response to hypercapnia (I group: 29 subjects) and the orthostatic test (II group: 17 subjects). We have found that hypercapnia inducesa significant increase of the flow velocities in the basal veins, on average 60 +/- 22.7% (p < 0.00001). The reaction prevents venous hyperaemia, i. e., significant increase of the cerebral blood flow volume and the drainage of the brain. In contrast to hypercapnia, the orthostatic test led to decrease in the flow velocities in the basal veins. The percentage of velocity decrease ranged from 16to 32% (p < 0.001). Posturally induced reduction of the venous flow volume prevents excessive drainage from the brain. Thus, we have shown that both tests can be used for evaluation of reactivity of the intracranial venous system and they can complement each other.