This was an open-label (both the investigator and the patient knew the identity of the study drug), nonrandomized pharmacokinetic (PK) study evaluating the interaction of topiramate and risperidone in patients with bipolar disorder. The study enrolled a sufficient number of patients to obtain 24 completed patients. Patients were men or women aged 18 to 55 years, inclusive, with a diagnosis of Bipolar Spectrum or Schizoaffective Disorder, who were not in the midst of an acute episode, and had not experienced an acute manic, major depressive, or schizoaffective episode for a minimum of 30 days prior to screening (enrollment was monitored to ensure that at least one-third of the study sample was of the same sex.) The study consisted of a screening phase and 3 treatment periods. In Period I, patients were stabilized to a clinically appropriate dose of risperidone within the range of 1 to 6 mg/day, administered in divided doses every 12 hours during a 2- to 3-week period (or longer as clinically necessary). Upon risperidone stabilization, serial blood and urine samples were obtained through 12 hours postdose for estimation of risperidone and its metabolite 9-OH-risperidone (metabolites are formed by metabolism of the drug) concentrations in Period I. In Period II, which lasted up to 6 weeks or longer as clinically necessary, topiramate was gradually escalated to 3 steady-state target doses, while risperidone therapy continued unchanged. There were up to 3 PK sampling periods (days when multiple blood and urine samples are taken to estimate the amount of topiramate and /or risperidone and its metabolite 9-OH-risperidone in blood or urine) when the patient achieved steady state at 100 mg/day topiramate (consistent amount of drug in blood with each dose); when/if the patient achieved steady state at 250 mg/day topiramate or maximum tolerated dose (MTD); and when/if the patient achieved steady state at 400 mg/day topiramate or MTD. During each PK sampling visit in Period II, serial blood and urine samples were obtained through 12 hours postdose for estimation of risperidone, 9-OH risperidone, and topiramate concentrations. In Period III, risperidone was gradually tapered while the 400-mg/day dose (or MTD) of topiramate was maintained. There were 2 PK sampling visits: when patients had attained steady state at a dose of 50% of the maximal risperidone dose reached in Period I; and when risperidone was discontinued for 7 days and patients were maintained on topiramate 400 mg/day or their respective MTD. During each PK sampling visit in Period III, serial blood and urine samples were obtained through 12 hours postdose for estimation of risperidone, 9-OH risperidone, and topiramate concentrations. Period I (risperidone stabilization): risperidone 1-6 mg/day for 2 to 3 weeks. Period II (topiramate dose escalation): topiramate administered for approximately 6 weeks titrated up to 3 potential target doses (100 mg/day, 250 mg/day and 400 mg/day), and risperidone continues unchanged. Period III (risperidone dose reduction): risperidone dose tapered to zero over a 4-week period while topiramate maintained at the target dose of 400 mg/day (or MTD).