Although clinical trials of high-dose methotrexate (MTX) sequenced before 5-fluorouracil (FUra) with leucovorin (LV) rescue apparently have resulted in increased numbers of tumor responses, this increased antitumor activity often has been accompanied with toxicity. The present report describes an attempt to improve therapeutic results with this drug combination by appropriate metabolic modulation in the preclinical BALB/c X DBA/8 F1 murine breast tumor model. A LV rescue schedule consisting of 300 mg/kg administered at 4.5 and 19.5 hr after high-dose MTX (300 mg/kg/week for 3 weeks) prevented MTX toxicity. When FUra was administered 2.5 hr after MTX (with LV rescue), the dose of FUra had to be decreased, and we could not obtain convincing evidence for a differential cytotoxic effect on tumor versus normal host tissue. However, when a delayed uridine rescue schedule was added to protect the host from the toxic activity of FUra, the FUra dose could be increased even in the presence of high-dose MTX, and the therapeutic result was enhanced significantly without an increase in host toxicity. Finally, it was possible to add N-phosphonacetyl-L-aspartate to this drug combination (in the appropriate sequence: N-phosphonacetyl-L-aspartate before high-dose MTX-before high-dose FUra, followed by double rescue with LV and uridine) without producing increased toxicity to yield a significant increase in partial tumor regression rate. The biochemical rationale for the selection and sequence of administration of these agents is discussed.