OBJECTIVES: I. Determine the efficacy of phenylacetate in terms of response rate and time to progression in children with recurrent or progressive brain tumors, or with previously untreated poor prognosis brain tumors. II. Assess the toxicity of phenylacetate in these patients treated at the maximum tolerated dose. III. Determine the correlation between serum steady state phenylacetate levels and toxicity or response in these patients.