Ethosuximide (ESM) was determined by the EMIT technique in tears, saliva, cerebrospinal fluid (CSF), and plasma of epileptic subjects. CSF/ plasma, saliva/plasma and saliva/CSF ratios were in good agreement with gasliquid chromatography (GLC). The tears/plasma ratio was not significantly different from the saliva/plasma ratio, although the standard deviation was higher in the latter. These data indicate that ESM can be determined in a quantitatively similar manner in both saliva and tears by EMIT.