We synthesized novel 15-16 nornaltrexone derivatives 9, 11 and 22 to examine the importance of the cavity in the Beckett-Casy model, which was proposed to interact with the 15-16 ethylene moiety in the morphine structure. All the synthesized compounds showed lower affinities for the opioid receptor than did the naltrexone (10). The binding affinities of 14-OH derivatives 11, in which the rotation of the 9-17 bond would be restricted by an intramolecular hydrogen bond, was improved compared to the corresponding 14-H derivatives 9. Compound 22 whose 9-17 bond was strictly fixed by the ethylene bridge hardly bound to the opioid receptor. Compound 26 also showed very weak binding affinity in spite of the existence of the 15-16 ethylene unit. We proposed an important role for the orientation of the lone electron pair on the 17-nitrogen rather than the significance of the cavity in the Beckett-Casy model.