To better understand structural requirements for a μ ligand of the trans-3,4-dimethyl-4-(3-hydroxyphenyl)-piperidine class to interact with the μ opioid receptor, we have described in the previous article (Le Bourdonnec, B. et al. J. Med. Chem. 2006, 25, 7278-7289) new, constrained analogues of the N-phenethyl derivative 3. One of the active constrained analogues, compound 4, exhibited subnanomolar μ-opioid receptor affinity (Ki = 0.62 nM) and potent μ-opioid antagonist activity (IC50 = 0.54 nM). On the basis of structure 4, a new series of μ-opioid receptor antagonists were designed. In these compounds the octahydroquinolizine template of 4 was replaced by an octahydro-1H-pyrido[1,2-a]pyrazine scaffold. The new derivatives were tested for their binding affinities and in vitro functional activity against the cloned human μ-, δ-, and κ-opioid receptors. From this study, we identified compound 36, which displays high affinity toward the μ-opioid receptor (Ki = 0.47 nM), potent μ in vitro antagonist activity (IC50 = 1.8 nM) and improved binding selectivity profile μ/κ and μ/δ, when compared to 4.