Due to unsatisfactory equipment efficiency and the time consuming manual procedures of sample preparation, drug analyses in physiological fluids and tissues frequently have to be carried out a few days after the sample collection. This is especially the case with investigations which require the examination of materials for which a large number of samples is necessary. The paper deals with the influence of storing blood samples on the level of propofol in blood and plasma. Propofol (2,6-diisopropylphenol, Diprivan) is a very popular intravenous agent used both for the induction and the maintenance of anaesthesia in human and veterinary patients as well as in laboratory animals. The results obtained show that, due to distinct losses of propofol in samples during their storage, the comparison of data estimated for subsequent days after sampling can lead to misleading or even wrong conclusions. The speed of drug diminution depends both on the type of blood and the anticoagulant used. The established interdependencies between the change in the level of propofol in blood and plasma samples and their storage time show that analogous investigations of other pharmaceutical agents are necessary.