The effect of lamotrigine on the steady-state plasma concentrations of the atypical antipsychotics clozapine, olanzapine, and risperidone was investigated in patients with schizophrenia or bipolar disorder stabilized on chronic treatment with clozapine (200-500 mg/day; n = 11), risperidone (3-6 mg/day; n = 10) or olanzapine (10-20 mg/day; n = 14)). Lamotrigine was titrated up to a final dosage of 200 mg/day over 8 weeks, and pharmacokinetic assessments were made at baseline and during treatment weeks 6 and 10, at lamotrigine dosages of 100 and 200 mg/day respectively. The plasma concentrations of clozapine, norclozapine, risperidone, and 9-hydroxy-risperidone did not change significantly during treatment with lamotrigine. The mean plasma concentrations of olanzapine were 31 +/- 7 ng/mL at baseline, 32 +/- 7 ng/mL at week 6, and 36 +/- 9 ng/mL at week 10, the difference between week 10 and baseline being statistically significant (P < 0.05). Adjunctive lamotrigine therapy was well tolerated in all groups. These findings indicate that lamotrigine, at the dosages recommended for use as a mood stabilizer, does not affect the plasma levels of clozapine, risperidone, and their active metabolites. The modest elevation in plasma olanzapine concentration, possibly due to inhibition of UGT1A4-mediated olanzapine glucuronidation, is unlikely to be of clinical significance.