During an infusion of thiopentone to control status epilepticus secondary to hypoxic brain damage, thiopentone and pentobarbitone levels were measured in the serum and cerebrospinal fluid (CSF). Pentobarbitone was found to be present in the serum between 7.8 and 11.1% of the thiopentone levels. There was a prompt response in serum levels of thiopentone to changes in the infusion rate. The CSF thiopentone varied between 15 and 40% of the serum levels. The CSF anticonvulsant threshold for thiopentone for this patient was between 5 and 15 mg/litre.