The fundamentally important question of whether morphine and its congeners adopt a conformation in the drug-receptor complex with the nitrogen substituent in an equatorial or axial configuration—critical for the rational development of novel agents targeting the opiate receptor—has not been resolved. Previous studies proposed that agonists require equatorial nitrogen substituents (Belleau et al.) and antagonists are distinguished by equatorial orientations (Feinberg et al.), but recent evidence indicates both axial and equatorial substituents can produce active agonists. Quantum mechanical calculations show small axial-equatorial minimum-energy differences for protonated (0.4 kcal/mol) and neutral (2.9 kcal/mol) morphine, comparable to ligand-receptor binding energy, raising uncertainty about the relevance of isolated molecule conformations to drug-receptor interactions. To address this, we examined the pharmacology of diastereoisomeric quaternary compounds A (axial N-allyl) and B (equatorial N-allyl), prepared by alkylating morphine with allyl iodide and N-allylnormorphine with methyl iodide, respectively. We tested their ability to displace [³H]diprenorphine from rat brain homogenate and evaluated μ/δ agonist/antagonist activity in the electrically stimulated mouse isolated vas deferens (mvd) and μ/κ activity in the guinea pig isolated ileum (gpi). Quaternization reduced antagonist potency against μ/δ agonists and binding affinity for [³H]diprenorphine: axial isomer A had ~1/100 nalorphine's antagonist potency in mvd (paralleling binding affinity decreases) and failed to antagonize Met⁵E at 22 pmol. Equatorial isomer B retained ~1/3 nalorphine's binding affinity and antagonist potency against normorphine in mvd but showed lower potency against Met⁵E. Isomer A exhibited weak gpi agonist activity (reversed by naloxone), while B was a pure antagonist. X-ray diffraction of isomer B definitively confirmed the equatorial configuration of the N-allyl group. The distinct pharmacological profiles of A (some agonist activity) and B (pure antagonist) support the hypothesis linking N-substituent conformation to agonism/antagonism. This study provides the first unequivocal experimental evidence that the equatorial configuration is the preferred conformation for the N-substituent in opiate antagonists.