The aim of the work was to demonstrate whether acute hypercapnia (paCO2 > 65 mmHg) influenced the permeability of BBCSF. 12 Chinchilla rabbits which underwent general anaesthesia were randomly divided into 2 groups. To maintain normal paCO2 values, the investigation was performed under normal ventilation in control group (No I). Controlled hypoventilation was applied to achieve acute hypercapnia in the shortest possible time in the investigated group (No II). BBCSF function was evaluated using gentamicin permeability indexes QG (defined as gentamicin concentration in the cerebrospinal fluid-to-serum gentamicin concentration in given moment of investigation). Mean haemodynamic and haematologic parameter values showed no significant within-group and between-group deviation. Statistically insignificant increase of QG values after the first hour of hypercapnia was found when comparing mean QG values in the investigated group. Mean QG value was statistically significantly higher in the animals hypoventilated for 3 hours when compared with animals which underwent normal ventilation during the whole procedure. Severe, acute hypercapnia disturbs the integrality of the blood/cerebrospinal fluid barrier.