Seric values have been evaluated in 24 patients receiving high doses of etoposide. In 6 patients, CNS value was evaluated simultaneously. After 2 perfusions with 250 mg/m2 at 12 h interval, seric value is 35.3 micrograms/ml + 10.3, the value after 6 perfusions is 31.1 micrograms/ml + 6.5. The variation of CNS value observed from 0.08 to 0.49 micrograms/ml is 0.36 to 1.29% of the seric value; there is no positive correlation with seric value. Only one patient out of 6 has a partial response of CNS metastases.