A new technique enabling repetitive sampling of CSF in unanesthetized rhesus monkeys was developed to study the pharmacokinetics of methotrexate (MT) in the CSF. CSF and plasma MTX levels were monitored following intraventricular and i.v. administration. CSF and plasma MTX disappearance curves in the monkey were identical to curves generated in humans, suggesting that the mechanisms of transport between the CSF and plasma compartments are similar in both species. These observations validate this experimental primate model and indicate its potential application to the pharmacological study of CNS [cancer] chemotherapeutic agents in man.