The functional in vitro study of the enantiomers of imidazolines 4-7 highlighted the role played by the nature of the ortho phenyl substituent in determining the preferred α(2C)-AR configuration. Indeed, the (S) enantiomers of 4-6 or (R) enantiomer of 7 behave as eutomers and activate this subtype as full agonists; the corresponding distomers are partial agonists. Because in clinical pain management with opioids α(2C)-AR agonists, devoid of the α(2A)-AR-mediated side effects, may represent an improvement over current therapies with clonidine like drugs, 4 and its enantiomers, showing α(2C)-agonism/α(2A)-antagonism, have been studied in vivo. The data suggest that partial α(2C)-activation is compatible with effective enhancement of morphine analgesia and reduction both of morphine tolerance acquisition and morphine dependence acquisition and expression. On the contrary, full α(2C)-activation appears advantageous in reducing morphine tolerance expression. Interestingly, the biological profile displayed by 4 (allyphenyline) and its eutomer (S)-(+)-4 has been found to be very unusual.