We have extended automated sequential trace enrichment of dialysates and high performance liquid chromatography to include the simultaneous assay of lamotrigine with other anticonvulsant drugs: phenobarbitone, carbamazepine and phenytoin. The procedure is reliable and precise (between-batch coefficients of variation < 2.8%) with no interference from other commonly prescribed drugs. Patients on lamotrigine presented with serum concentrations up to 67 mumol/L with no adverse effects, suggesting that the therapeutic range may need re-appraisal.