The effects of fentanyl, ketamine and thalamonal on some biochemical parameters of ethanol-treated and untreated dogs, were studied. The study was carried out on mongrel dogs. The animals were divided into two main groups: one which had free access to food and to 12% (v/v) ethanol, instead of water, and a control group which had food and water ad libitum. The animals in both groups were exposed to the action of anaesthetics for 3 h. The results show that, under the given experimental conditions, the anaesthetics did not significantly change the investigated parameters of blood and liver, either in ethanol-treated or untreated dogs. However, the action of ethanol caused remarkable changes in almost all the investigated parameters. An increased content of cytochrome P450 caused by the action of ethanol suggests possible changes in the metabolism of anaesthetics, which should be of concern to anaesthetists when dealing with patients potentially intoxicated with ethanol.