Cyclo(Leu-Gly) (CLG), a diketopiperazine analog of Pro-Leu-Gly-NH2 (MIF), has direct effects on dopamine (DA) mediated behaviors as well as on D-2 DA receptors. Endogenous opioids, as well as morphine have also been implicated as neuromodulators of dopaminergic function. We studied these interactions in an animal model in which chronic morphine administration induces a dopaminergic supersensitivity that can be detected during the 48 hour (h) period following withdrawal of morphine. At 24 h following morphine withdrawal, there was a 3.5-fold increase in stereotypic behavior in rats following a challenge dose of apomorphine (APO) (0.5 mg/kg). By 48 h this effect had disappeared. Co-administration of CLG (8 mg/kg s.c.) with morphine attenuated the development of the behavioral supersensitivity to APO. D-2 DA receptor binding analysis indicated that parallel molecular changes occurred. There was a morphine-induced increase in the affinity (+167 percent) in antagonist (i.e. 3H-spiroperidol displaced by butaclamol) binding at 24 h after withdrawal. Co-administration of CLG with morphine attenuated these DA receptor changes at 24 hours which is consistent with the peptide's effect on stereotyped behavior. However, antagonist binding parameters did not parallel changes in behavior at 48 h. Agonist binding was then studied by examining DA displaceable 3H-spiroperidol (75 pM) binding to the D-2 DA receptor. Two receptor subpopulations D-2-HI and D-2-LO were revealed. Morphine caused an increase in the affinity for agonist binding to the D-2-HI site (83-fold increase). Affinity changes at the D-2-HI site correlated positively and strongly with the behavioral changes in all groups at both 24 and 48 h. We conclude that changes in agonist binding to D-2 DA receptors rather than antagonist binding is more consistent with the behaviors induced by morphine and CLG.