The mechanism of development of postoperative pain is complex. Central and peripheral sensitization are playing an important role and this can lead to postoperative hypersensitization. Several studies have shown, that S (+) ketamine can be effective to reduce sensitization and postoperative pain. Ketamine (2-O-chlorophenyl-2-methylamino cyclohexanone) is a N-Methyl-D-Aspartat (NMDA) receptor antagonist. S (+) ketamine has a four times stronger affinity to the NMDA receptor compared to R (-) ketamine. The duration of action for S (+) ketamine is shorter than R (-) ketamine and it has fewer side-effects.