The primary objective of this Phase I study in 15 adult subjects suffering from epilepsy and chronically treated with phenytoin monotherapy is to evaluate the effect of steady-state ucb 34714 administration on the steady-state plasma levels of phenytoin.

Inclusion Criteria:

• Well-characterized epileptic syndrome according to the ILAE classification.
• Patients currently treated with stable phenytoin monotherapy for at least three months and with at least one plasma measurement of pheyntoin within the target range (7 - 23 µg/ml) during the screening period.

Exclusion Criteria:

• History of status epilepticus in the last year.
• Subjects taking any drug that may significantly influence the metabolism of ucb 34714 (CYP2C or CYP3A potent inducers/inhibitors) except if the dose has been stable at least three months before entry into the study and will be kept stable for the entire trial duration.
• Subjects with a creatinine clearance of ≤50 mL/min.