The study was designed to include 3 parts; only the first part is described in this report. Part I of the study was an international, randomized, placebo-controlled, double-blind, parallel group, multicenter clinical study conducted in 19 countries at 173 sites in 653 subjects with refractory simple partial or complex partial seizures, with or without secondary generalization. After screening procedures and confirming eligibility, subjects entered Part I of the study, which consisted of 3 periods.