An unbiased preference conditioning procedure was used to characterize and compare the motivational effects of opioids in naive rats and those suffering from the prolonged pain associated with Freund's adjuvant (FA)-induced inflammation of one hind limb. The mu-opioid agonist morphine functioned as a reinforcer in naive animals, producing marked preferences for the drug-paired place. Similarly, rats injected with FA 7 days prior to conditioning exhibited a preference for the morphine place, and the magnitude of this effect did not differ between groups. Administration of the kappa-opioid receptor agonist U-69593 to naive rats produced dose-related place aversions. The aversive effect of this kappa-agonist was, however, abolished in FA-treated rats. Thus, regardless of the dose administered, U-69593 produced conditioning similar to that observed in response to saline. These data suggest that kappa-agonists may lack aversive effects in subjects experiencing prolonged noxious stimulation, and as such may be effective therapeutic agents in the management of chronic pain states.