Behavioural sensitization involves progressive increases in behavioural responses to repeated intermittent administration of drugs of abuse. Behavioural sensitization is observed to the locomotor stimulant, rewarding and discriminative effects of a drug. These are effects which seem to be essential in the initiation, expression and maintenance of a drug-seeking behaviour. Therefore the phenomenon of behavioural sensitization may have important implications for the understanding of addictive processes. Findings given in this review demonstrate the involvement of endogenous opioid systems in the initiation of sensitized responses on the neurochemical level, i.e., within the mesolimbic dopaminergic system, as well as on the behavioural level. Specifically, it is shown that behavioural sensitization to morphine and cocaine is modulated by endogenous kappa-opioid systems.