Spiro derivatives of 4-phenylpiperidine analgesics would appear to be interesting receptor probes inasmuch as the rotation of the phenyl ring is restricted in such compounds. A number of examples have appeared in the literature and the importance of the torsional relationship between the aromatic group and the piperidine ring in nonspiro analgesics has received attention. ln-'Although biological data are not abundant, in general, spiro ...