Phenylbutyrate is an aromatic fatty acid that is converted to phenylacetate in vivo by mitochondrial beta-oxidation to phenylacetate. Preclinical studies have shown that continuous exposure to phenylacetate or phenylbutyrate can induce tumor cytostasis and differentiation in a wide variety of cell lines including malignant gliomas and neuroblastomas. However, phenylbutyrate has been shown to be a more potent differentiating agent than phenylacetate in a variety of tumor cell lines. In addition, phenylbutyrate appears to have molecular activities that are distinct from phenylacetate. The objective of this trial is to determine the maximum tolerated dose and the toxicities of phenylbutyrate administered as a continuous intravenous infusion for 28 days. In addition, the pharmacokinetics of phenylbutyrate and its metabolite, phenylacetate, will be studied using both model-dependent and model-independent parameters.