OBJECTIVES: I. Determine the efficacy of high-dose methotrexate (HMTX) in combination with mechlorethamine, vincristine, prednisone, and procarbazine (MOPP) in infants or young children with primitive neuroectodermal tumors (PNET) (including medulloblastoma, anaplastic ependymoma, ependymoblastoma, or pineoblastoma) or high-grade astrocytoma. II. Determine whether the addition of HMTX to MOPP (MMOPP) improves the continuous complete response rate of MOPP alone and eliminates the need for salvage with radiotherapy in these patients. III. Determine the ability of MMOPP to provide neuroaxis prophylaxis or to treat spinal metastasis without radiotherapy in infants or young children with PNET. IV. Determine the toxicity of this regimen in terms of neurologic and neuropsychologic sequelae, growth, and development in these patients. V. Correlate the efficacy of this regimen with the histopathologic diagnosis of these patients. VI. Determine the optimum method for radiographic evaluation of spinal cord disease in patients with PNET. VII. Determine the utility of sequential spinal cord radiography as a means of monitoring PNET in these patients.