Abstract
Methotrexate pharmacokinetic studies, performed on a patient with renal impairment who had toxic effects following 20 mg/sq m of intrathecally administered methotrexate, demonstrated prolonged serum concentrations of the drug, which accounted for the condition. After the return of normal renal function, pharmacokinetic studies were repeated following the same dose and route of administration of methotrexate. On this occasion there was a rapid clearance of serum methotrexate below toxic levels.