The toxicity of the glutamine antagonist 6-diazo-5-oxo-L-norleucine (DON) administered as a 24 hour infusion has been evaluated. Studies of the clinical pharmacology of the drug have also been performed in 3 patients. The limiting toxicity of the drug was acute nausea, vomiting and diarrhea that was dose dependent in its severity and duration. The maximum tolerated dose was 600 mg/m2 over 24 hours. The other major toxicity was thrombocytopenia that was maximal 7-10 days after the completion of the infusion. The drug does not exhibit renal, hepatic or central nervous system toxicity. DON achieves steady state levels during these infusions and is eliminated by first order kinetics when the infusion is completed (t1/2 alpha = 1.81 h). The principal route of excretion is renal. A starting dose of 400 mg/m2 would be acceptable for Phase II studies of this drug administered on this schedule.