We investigated the rate of penetration into and the intra-relationship between the serum, cerebrospinal fluid (CSF) and regional brain extracellular fluid (bECF) compartments following systemic administration of lamotrigine in rat. The serum pharmacokinetics were biphasic with an initial distribution phase, (half-life approximately 3 h), and then a prolonged elimination phase of over 30 h. The serum pharmacokinetics were linear over the range 10 - 40 mg kg(-1). Using direct sampling of CSF with concomitant serum sampling, the calculated penetration half-time into CSF was 0.42+/-0.15 h. At equilibrium, the CSF to total serum concentration ratio (0.61+/-0.02) was greater than the free to total serum concentration (0.39+/-0.01). Using in vivo recovery corrected microdialysis sampling in frontal cortex and hippocampus with concomitant serum sampling, the calculated penetration half-time of lamotrigine into bECF, 0.51+/-0.11 h, was similar to that for CSF and was not area or dose dependent. At equilibrium, the bECF to total serum concentration ratio (0.40+/-0.04) was similar to the free to total serum concentration (0.39+/-0.01), and did not differ between hippocampus and frontal cortex. The species specific serum kinetics can explain the prolonged action of lamotrigine in rat seizure models. Lamotrigine has a relatively slow penetration into both CSF and bECF compartments compared with antiepileptic drugs used in acute seizures. Furthermore, the free serum drug concentration is not the sole contributor to the CSF compartment, and the CSF concentration is an overestimate of the bECF concentration of lamotrigine.