Carbamazepine (CBZ) was determined by EMIT assay in plasma, cerebrospinal fluid (CSF), and brain samples of cats after administration of the drug (40 mg/kg, i.p.). Plasma and CSF levels increase in a parallel manner from time 0 to 90 min after completion of injection, indicating a passive transport from one body fluid to the other. Brain levels reach a steady state at 15 min, with no significant difference being found between the concentrations at 15, 30, 60, and 90 min postinjection; this indicates that substantial early binding occurs in the brain. Since peak CSF concentrations occur at 90 min, concomitant with those for plasma, while brain levels are already high at 15 min, it is likely that CBZ enters the two compartments by independent mechanisms.