Naltrindole-5'-isothiocyanate (5'-NTII), a nonequilibrium delta opioid receptor antagonist, has been shown to antagonize differentially the antinociception induced by DSLET without affecting that induced by DPDPE. In the present study, we investigated whether or not 5'-NTII can differentially affect DSLET and DPDPE binding to receptor sites in striatal slices of mouse brain. We found that 5'-NTII only changed the binding characteristics of [3H]DSLET and not that of [3H]DPDPE. Saturation binding studies revealed that 5'-NTII treatment in vitro inhibited [3H]DSLET binding by decreasing the affinity but not the number of binding sites. This finding was supported by saturation studies with [3H]DSLET in striatal slices from mice that were pretreated with 5'-NTII (10 nmol, i.c.v. or 10 mg/kg, s.c.). Thus, the results suggest that 5'-NTII can antagonize differentially the binding to striatal slices of mice of [3H]DSLET but not that of [3H]DPDPE. The binding parameters also suggest that 5'-NTII may not antagonize [3H]DSLET binding by alkylating at the receptor recognition site because the number of binding sites did not decrease. 5'-NTII may bind to some other part of the membrane to indirectly desensitize the receptor to a low affinity form. Lastly, the differential alteration of binding sites between [3H]DSLET and [3H]DPDPE by 5'-NTII strongly support the postulated existence of delta opioid receptor subtypes.