Levels of methotrexate (MTX) measured by both 3H radioactivity and dihydrofolate reductase assays were determined in cerebrospinal fluid (CSF), plasma, urine, and both neural and non-neural tissues at varying times after a single intraventricular injection into Cynomolgus monkeys (Macaca fascicularis). Clearance of the MTX from CSF was rapid after injection. A relatively constant level of 3HMTX was reached in plasma 2 1/2 hours after injection, and about 30% of the 3HMTX dose was excreted in the urine within 4 hours after injection. Maximum levels in CNS tissues were obtained by 4 hours after injection, and average concentrations of 10(-6) M MTX (moles/kg wet weight) were maintained in CSF for up to 12 hours and in brain for up to 24 hours after injection. Conversion of MTX to non-MTX products was detected in CSF between 4 and 12 hours, and in brain tissue between 12 and 24 hours after injection, and the amount of these products increased with time. Regional distribution studies in the cerebrum showed a U-shaped distribution curve for 3HMTX up to 12 hours after injection, which closely followed the 14C inulin distribution. Thus, the levels in deep cerebral tissue were less than the average level for brain, and this suggests that treatment of CNS tumors by intraventricular injection may have variable results, partly due to complex tissue distribution patterns.