We report a case of interaction between anticonvulsant and antineoplastic drugs in one male patient with seizures from brain tumour. The patient was treated with phenytoin (PHT), phenobarbital (PB), and an antineoplastic protocol based on a combination regimen with carmustine (BCNU), vinblastin (VLB), methotrexate (MTX), and radiotherapy. Plasma concentrations of PHT fell from 9.4 to 5.6 micrograms/ml 24 h after VLB administration, and remained low for at least 10 days. During this period, partial seizures occurred. Plasma concentrations of PB were unchanged during the period of observation. It is suggested that impaired absorption of PHT, caused by VLB or MTX or both, is responsible for this interaction.