Study participants will have a continuous infusion of phenylbutyrate for two 28-day cycles-every day, 24 hours a day, 7 days a week. The medicine will be infused through a thin tube (catheter) placed in a large vein in the upper chest, delivered through a portable infusion pump. Patients will be hospitalized for at least 3 days when the treatment begins. If there are no side effects at that time, the infusions can continue on an outpatient basis. The patient or care giver will receive the medicine in 4-day supplies and will be taught how to change the bag and tubing daily for drug administration, as well as how to use the infusion pump. Patients will be monitored with weekly blood tests to look for side effects and measure blood levels of phenylbutyrate. They will have a physical examination at least once a week. At the end of the second 28-day cycle, patients will have a CT or MRI scan to evaluate the tumor's response to treatment. Patients whose tumor has grown will stop treatment and come off the study. Those whose tumor has remained stable or shrunk may continue phenylbutyrate as long as the treatment is beneficial and there are no serious side effects. CT or MRI scans will be done after every 2 cycles (or sooner if needed) to evaluate the treatment.