The series of experiments reported in this paper examined the spectrum of subtypes of opioid receptors alkylated in vitro by N-cyclopropylmethyl-7 alpha-methylfumaramido-6,14- endoethenotetrahydronororipavine (NIH10236) and four optical isomers of the methylfumaramidophenethyl derivatives of 3-methylfentanyl. Pretreatment of membranes with NIH10236 resulted in a wash-resistant inhibition of the binding of [3H]6 beta-fluoro-6-desoxyoxymorphone (mu binding sites), the binding of [3H][D-ala2,D-leu5]-enkephalin (both the higher and lower affinity delta binding sites) and was without effect on kappa binding sites labelled with [3H]bremazocine. All four potential alkylating derivatives of 3-methylfentanyl were inactive. Pretreatment of membranes with 1 microM of the reversible ligands, (+)-cis-3-methylfentanyl, but not its enantiomer, inhibited the binding of [3H]6 beta-fluoro-6-desoxyoxymorphone and the binding of [3H][D-ala2,D-leu5]enkephalin to the lower affinity binding sites by over 90%. This phenomenon is termed "pseudo-irreversible inhibition." Incubation of pretreated membranes for 60 min at 37 degrees C, in the presence of 200 mM NaCl and 50 microM GppNHp, only partially reversed the masking of opioid receptors by (+)-cis-3-methylfentanyl. For in vivo experiments, membranes were prepared 18-24 hr after the intracerebroventricular administration of 80 and 50 micrograms of NIH10236. This resulted in decreased labelling of mu binding sites, lower affinity [3H][D-ala2,D-leu5]enkephalin binding sites, as well as kappa binding sites, labelled by [3H]U69,593 and [3H]bremazocine. There was no apparent alteration in the higher affinity [3H][D-ala2,D-leu5]enkephalin binding site.(ABSTRACT TRUNCATED AT 250 WORDS)