N-[1-(2-Hydroxy-2-phenylethyl)-3-methyl-4-piperidyl]-N-phenylpropanamide (ohmefentanyl,1) is an extremely potent analgesic agent with high affinity and selectivity for opioid mu receptors. There are three chiral carbons in 1, so eight optically active isomers are possible. Respective reaction of optically active 3-methyl-N-phenyl-4 -piperidinamines (5a-d) with (R)- or (S)-styrene oxide produced eight optically active intermediates which were subsequently converted to eight optically active isomers of 1 (1a-h). The absolute configurations of 1a-h were determined by X-ray analysis of (3R,4S,2'R)-(-)-cis-1a and (3R,4R,2'S)-(-)-trans-1g. The analgesic activity (mice, ip, hot plate) revealed their extreme stereodifferences; the ED50 values of (3R,4S,2'R)-(-)-cis-1a and (3R,4S,2'S)-(+)-cis-1b, which are the most potent isomers among eight isomers, were 0.004 65 (2990 times that of morphine) and 0.001 06 mg/kg (13 100 times that of morphine), respectively, while the corresponding antipodes 1d,c were the least potent compounds among the eight isomers. In agreement with pharmacological results, both 1a,b also had the highest receptor affinity and selectivity for the opioid mu receptor. The ratio of K(i)(DPDPE)&K(i)(DAMGO) was 22 800 for 1a and 22 500 for 1b. All isomers except 1c,d strongly inhibited the electrically evoked smooth muscle contraction of GPI and MVD but not that of RVD, and the inhibitory effects could be reversed by naloxone, which indicated that these compounds were potent mu agonists in GPI and MVD. There was a good linear correlation between the analgesic potencies (ED50) and the receptor affinities (K(i)(DAMGO)) or inhibitory effects (IC50) to GPI and MVD. These results suggested that the analgesic effects of ohmefentanyl are mediated by interaction between the agents and opioid mu receptors in the central nervous system and the 3R,4S configuration at the piperidine 3- and 4-carbon atoms and the S configuration at the phenylethyl 2-carbon atom are beneficial for analgesic potency and inhibitory effects in GPI and MVD and the same for an S or R configuration at the phenylethyl 2-carbon atom besides the 3R,4S configuration for receptor mu affinity and selectivity.