Clinical observations show that there is considerable individual variability in the response to the addictive properties of drugs. This individual variability needs to be taken into account in animal models of addiction. Like humans, only some rats readily self-administer low doses of psychostimulants. The individual animals at risk can be identified on the basis of their response to environmental or pharmacological challenges. This predisposition to develop self-administration can be induced by repeated treatment with amphetamine. These results may help elucidate the neurobiological basis of addiction liability observed in both rats and humans.