Four separate groups of patients have been studied: (1) The effect of high-dose methotrexate (MTX) administration on glomerular filtration rate was determined by pre- and posttreatment inulin and creatinine clearances in nine patients. Measurements were made prior to and 24-40 hr after drug administration. Inulin and creatinine clearances both decreased a mean of 43%. No signs of systemic toxicity occurred. (2) Three other patients given high-dose courses of MTX developed MTX toxicity. Their creatinine clearance decreased an average of 61%. (3) In a separate group of five patients undergoing weekly MTX treatment, comparison of serum MTX pharmacokinetics with and without alkalinization of the urine demonstrated no significant difference in peak serum MTX levels or serum MTX decay. (4) Eight additional patients with severe renal dysfunction secondary to MTX were treated with increased doses of leucovorin and a continuous infusion of thymidine (8 g/m2/day) once renal failure was recognized. When high-dose leucovorin and thymidine were begun 48-72 hr after the MTX infusion, severe toxicity in the form of leukopenia, thrombocytopenia, diffuse mucositis, stomatitis, or skin rash was averted. We concluded the following: (1) high-dose MTX causes a subclinical decrease in glomerular filtration rate with each administration, even in nontoxic courses; (2) alkalinization of the urine with sodium bicarbonate does not alter plasma MTX decay, while volume expansion (hydration) is maintained constant; and (3) rigorous monitoring of serum creatinine and serum MTX levels 24-48 hr after MTX administration allows for the institution of rescue measures, including leucovorin and thymidine, which will abort the systemic toxicity that accompanies MTX-induced renal failure.