A place preference conditioning procedure was used to characterize the motivational effects of β-endorphin-(1-27), a naturally occurring fragment of β-endorphin (β-EP). The intracerebroventricular (ICV) administration of β-EP, selective μ-(DAGO) or δ-(DPDPE) opioid receptor agonists to rats produced marked preferences for the drug-associated place, whereas the selective κ-opioid receptor agonist, U-50488H produced conditioned aversions. ICV injections of the β-EP-(1-27) (5–20 μg), however, resulted in no preference for either the drug- or vehicle-associated place. Pretreatment with β-EP-(1-27) (10 μg) eliminated the place preference produced by β-EP. It abolished the place preferences induced by both DAGO and DPDPE but did not modify the effects of either U-50488H or the psychostimulant d-amphetamine. These data demonstrate that β-EP-(1-27) selectively antagonizes the motivational effects of μ- and β-opioid agonists and suggest that this fragment may function as an endogenous antagonist of the reinforcing effects of opioid agonists in vivo.