OUTLINE: Patients undergo maximum tumor debulking. Patients who have undergone incomplete resection proceed to induction. Patients with a primary diagnosis of primitive neuroectodermal and pineal tumors or glioblastoma multiforme who have undergone total resection proceed to induction. Induction: Patients receive high dose methotrexate (HMTX) IV over 6 hours on day 1. Beginning 3 hours after completion of HMTX infusion, leucovorin calcium (CF) is administered IV over 30 minutes every 3 hours for 9 doses. Beginning 3 hours after completion of the last CF infusion, oral CF is administered every 6 hours for 8 doses. Patients receive a second HMTX infusion beginning 1 week after completion of the first HMTX infusion. Beginning 1 week after completion of the second HMTX infusion, patients receive mechlorethamine IV and vincristine IV on days 1 and 8, oral procarbazine and oral prednisone on days 1-10, and tapered doses of prednisone on days 11-13 (MOPP). Maintenance: Beginning 4 weeks after initiating the first course of MOPP, patients receive HMTX on day 1 and MOPP beginning on day 4. Treatment continues every 31 days in the absence of disease progression or unacceptable toxicity. After 1 year or 14 doses of HMTX, whichever occurs first, HMTX is discontinued and treatment with MOPP alone continues every 4 weeks in the absence of disease progression or unacceptable toxicity. Treatment is discontinued after 2 years if the patient is in continuous complete remission.