Endogenous opioid systems have been implicated in experimental cocaine addiction. One aspect of this involvement may be the modulation of the motivational properties of cocaine by endogenous opioids. The present study assessed the effect of opioid blockade with naloxone (NLX) on cocaine's motivational properties using the conditioned place preference procedure. Treatment with doses of NLX that did not induce place aversion (0.01-1.0 mg/kg-1, SC), dose-dependently attenuated place preference induced by cocaine (10 or 20 mg/kg-1, IP). This effect of NLX was present when administered during acquisition of cocaine-induced place preference and when administered before expression of cocaine's motivational effects. These data support the notion that the (conditioned) motivational properties of cocaine are modulated through activation of opioid systems by endogenous opioid peptides. Furthermore, it is suggested that an interaction between endogenous opioid systems and dopaminergic systems in the brain might be of importance in the motivational facilitation of experimental cocaine addiction.