A pharmaco dynamic study of Methotrexate (MTX) during intraperitoneal MTX/5-FU sequential therapy was carried out after gastric surgery. A comparative study of the route of MTX administration and its dose was also done. 1) Comparative study of the serum concentration of MTX between i.p. and i.v. administration revealed a similar MTX concentration except immediately after administration. 2) A comparative study of the serum concentration of MTX administered i.p. between patients with and without malignant ascites was conducted. Immediate elevation of the serum concentration of MTX was observed in patients without malignant ascites. On the other hand, the MTX concentration was slowly elevated and washed out in patients with malignant ascites. 3) The MTX concentration in the intraperitoneal fluid was compared between patient with and without malignant ascites. In patients without malignant ascites, MTX disappeared quickly from the intraperitoneal fluid. However, the MTX concentration lasted long in the malignant ascites cases. These results were similar with a low-dose MTX (30 mg/body) or moderate dose (100 mg/body). MTX/5-FU sequential i.p. therapy can thus be an effective treatment for patients after gastric surgery, though clearance of MTX was slow in cases with malignant ascites.