The aim of this study was to compare physiological responses in Siberian sturgeon (Acipenser baeri) induced by propofol and CO2 anaesthesia. Two procedures were applied during the experiment. In procedure I, blood samples were collected immediately after exposure (1, 2, 5, 10 min) to the anaesthetic. In procedure II, fish were exposed to the anaesthetic for 10 minutes and then were moved to anaesthetic free water. Blood was sampled after 5, 10, 20 or 30 min of recovery time. Gasometrical and biochemical analyses were performed on collected blood. In CO2 anaesthetized fish strong hypercapnic acidosis was revealed. The drop of the HCO3-/CO2 ratio, from 28:1 in control fish up to 4:1 in CO2 anaesthetized ones, proved that the compensation mechanism is not capable of preventing acidosis during CO2 anaesthesia in Siberian sturgeon. In contrast, only moderate, respiratory acidosis occurred in sturgeons anaesthetised with propofol. Hypercapnic acidosis during CO2 anaesthesia was followed by a fourfold increase of ammonia level in the blood. Glucose level, increasing only during recovery time, indicates that a secondary stress response occurred when awareness of anaesthetized fish had been restored.