According to the World Health Organization (WHO), epilepsy afflicts more than 50 million people worldwide. Despite the ongoing use of older antiepileptic drugs (AEDs) and the development of newer treatments that are better tolerated, approximately 30% of patients, particularly those with partial seizures, are not well controlled even on newer treatments, or experience significant side effects from treatment. RWJ-333369 is a drug with anticonvulsant activity that is being investigated for the treatment of epilepsy. This is a randomized (patients are assigned different treatments based on chance), double-blind study (neither the patient nor the physician knows whether drug or placebo is being taken, or at what dosage ) in males and females who have partial onset seizures that have had an inadequate response to at least one AED. The study consists of 3 phases: pretreatment (a screening visit and a 56-day baseline period), double-blind treatment (12 weeks of treatment with either 200 mg per day of RWJ-333369, 400 mg per day of RWJ-333369, or placebo), and posttreatment (a posttreatment visit that occurs 7 to 14 days after the last dose of double-blind study drug). The posttreatment phase is only for patients not continuing in the open-label extension study. The open-label extension study is offered after completion of the double-blind treatment phase if the study doctor judges that the patient may benefit from continued treatment with RWJ-333369. The open-label extension study lasts until RWJ-333369 becomes available by prescription or its development is stopped by the sponsor. The efficacy of the RWJ-333369 will be based on a change in the frequency and severity of seizures. Safety assessments include adverse events (side effects) reporting, collecting blood tests and Electrocardiograms and performing physical exams, including vitals signs. The study hypothesis is that 400 mg per day of RWJ-333369 is better than placebo as add-on treatment of partial onset seizures, as measured by the percent reduction from baseline in the monthly partial onset seizure frequency. 200 mg per day RWJ-333369, 400 mg per day RWJ-333369, or placebo, given twice daily with or without food approximately 12 hours apart; study drug should be swallowed whole and not be chewed, divided, crushed, or dissolved.