To determine the maximum tolerated dose of sodium phenyllbutyrate taken orally three times a day in patients with refractory CNS tumors until evidence of tumor progression. To describe the pharmacokinetic parameters of oral phenylbutyrate and to escalate the does of phenylbutyrate in an attempt to achieve plasma levels of 2-6 mmol/L while evaluating patient tolerability on the continuous oral exposure schedule. To seek preliminary evidence of therapeutic activity of phenylbutyrate when administered on this schedule to patients with refractory CNS tumors. To correlate any observed responses and toxicity with results of bioassays and tissue sampling for phenylbutyrate activity.