Two strains of rats — LC2-Hi and LC2-Lo — selected for high and low self-stimulation rates, respectively, were tested for responses to opiates and to naloxone using conditioned place preference paradigm. In the two experiments which used opiates as UCS, conditioning was carried out in the non-preferred compartment while in the experiment which used naloxone, conditioning was performed in the preferred compartment. The preference changes were determined on the basis of times spent in the compartments before and after conditioning with drugs. LC2-Hi rats showed positive changes in the preference to the initially non-preferred side when morphine or heroin (5 mg/kg and 1 mg/kg, respectively) were used; no such effect was observed with LC2-Lo rats. Both lines exhibited aversive reactions to naloxone by diminishing the time spent in the environment paired with this drug, but again the response of LC2-Hi animals was significantly larger than the response of LC2-Lo rats. Chronic intake of a sweet solution (3 mM saccharin for 4 weeks) tended to amplify the aversive reaction to naloxone in both lines. It may be inferred from the present findings that there exists a common genetic factor, as revealed by the conditioned place preference paradigm, underlying positive reinforcing properties of opiates and aversive effects of naloxone.