Abstract: High-dose of methotrexate (MTX) was administered with citrovorum factor (CF) to 20 patients with malignant tumor. The pharmacokinetics and toxicities of MTX were studied following dosage of 17～62 mg/kg by a 4-hour infusion. Peak plasma MTX was attained just after the termination of drug infusion and the plasma levels fell rapidly thereafter. The pharmacokinetic parameters of the drug Were estimated by a nonlinear least squarer egression program. The results indicate that the plasma disappearance was a triphasic disintegration with half-lives of 0.365±0.09 hr (π), 1.43±0.22 hr (α) and 8.24±2.36 hr (β) (mean±SD). The formula of MTX pharmacokinetics was refined to provide plasma concentration over 24 hr. The cumulative urinary MTX excretion was 41.7%±1.8 of the administered dose. A low urine volume always resulted in a delayed drug excretion. High-dose MTX therapy with CF rescue was well tolerated for all patients. The toxicities of the drug were nausea, vomiting, elevation of serum transaminase level and a moderate leukopenia. The results show that a plasma MTX level of 5.4×10^-6 M at 24 hr or 6.21×10^-7M at 48 hr was a safe level.