A pharmacokinetic study of methotrexate (MTX) administered by the intravenous (IV) and intraventricular (via an Ommaya reservoir) route was performed in 16 children, 13 with acute lymphocytic leukemia (ALL) and three with non-Hodgkin's lymphoma. Five children with ALL were treated "prophylactically" for presumed subclinical central nervous system (CNS) leukemia. The remaining 11 patients were treated for overt meningeal involvement. MTX was administered intravenously at a dose of either 500 mg/m2 or 1500 mg/m2 with one third by rapid intravenous infusion and two thirds intravenously over 24 hours with leucovorin rescue. Intraventricular MTX was given in some treatment courses at a dose of 12 mg/m2. At either 500 mg/m2 or 1500 mg/m2, when given only IV, MTX results in a 100 fold higher concentration in plasma compared to cerebrospinal fluid (CSF). The plasma levels are three times higher with the 1500 mg/m2 dose compared to the 500 mg/m2 dose. After intraventricular administration of MTX, patients with overt CNS leukemia retained MTX in the CSF significantly longer than patients treated prophylactically. This may be due to abnormal transport of MTX out of the CSF in patients with meningeal disease.