Abstract: The crystal structures of methyl 4-N-(cyclopropyl-carbonyl)-N-phenylamino--1(2-phenylethyl)-4-piperidinecarboxylate (R32767) and N-4-acetyl-1-(2-phenylethyl)-4-piperidinyl-N-phenylpropanamide (R 32557) have been determined as part of a structure-activity relationship study on a series of 4-substituted fentanyl. R32767: C₂₅H₃₀N₂O₃; triclinic, pl; a=10.280(2),=b=10.367(1), c=12.170(1)A, α=92.28(1), β=102.31(1), γ=116.83(1)°, Z=2. R32557: C₂₄H₃₀N₂O₂·HCL·2H₂0; triclinic, pl; a=9.369(6), b=9.680(6), c=14.799(9)A, α=86.24(5), β=76.23(5), γ=68.70(5)°, Z=2. The structures of the two are compared with those of R30490, R30730 and R4263. Preliminary study indicates that its toxicity is concerned with angles between the two aromatic rings and the piperidine ring. The potent analgesic activity may correspond to a torsion angle C₁₄-C₁-N₂-C₁9 (according to locants of R32557) of about 120°.