The procedure involves operant conditioning of a preference for a particular environment that has been consistently paired with a subjective internal state induced by the tested substance or condition. If a drug has marked rewarding properties, the animal will spend more time in the compartment with which it was paired when subsequently tested without the drug. The conditioned place preference procedure is classically used for a long time to test the addictive liability of putative drugs of abuse in research as well as in pharmaceutical industry. The procedure may also be modified to determine whether genetically modified animals are more or less sensitive to the reinforcing effects of a drug.