The role of beta-endorphin as a possible mediator in the reinforcing properties of opiates was investigated using a conditioned place preference paradigm. Heroin, a synthetic opiate known to have reinforcing properties, produced a strong preference for an environment previously paired with heroin injection at all doses tested (0.25, 0.5, 1.0, 2.0 mg/kg SC). No such place preference was observed following saline injections. Rats also showed dose-dependent place preference for the environment paired with beta-endorphin when injected intracerebroventricularly (significant dose was 2.5 micrograms). At higher doses (5.0 and 10.0 micrograms) rats showed no preference for the paired environment, but were catatonic. Pretreatment with naloxone (0.04, 0.2, 1.0 mg/kg SC) attenuated the rewarding effect of beta-endorphin (2.5 micrograms) at all doses tested. The lowest dose of naloxone which had no aversive effect when tested alone could also significantly block the positive effect of beta-endorphin. The reinforcing dose of beta-endorphin (2.5 micrograms) also produced an increase in locomotor activity, when tested in photocell cages. This suggests that the hyperactivity induced by beta-endorphin may contribute to the preference for an environment previously paired with the same drug. The reinforcing effect of beta-endorphin is most probably mediated by the mu and/or delta opioid subtype receptor, since beta-endorphin has a high affinity for these receptors. These results demonstrate positive reinforcing properties of beta-endorphin in the central nervous system.