Clinically, 5-formyltetrahydrofolate (leucovorin, folinic acid, LV) in combination with 5-fluorouracil (5-FU) has been used at various doses, schedules, and routes of administration with therapeutic benefit to patients with advanced colorectal carcinoma and breast carcinoma. Clinical experiences have been primarily with LV doses of 25, 200, and 500 mg/m2 administered by either short-term intravenous infusion, daily continuous infusion, or orally. In patients with lung carcinoma, oral administration of dl-LV at 125 mg/m2 hourly for 4 hours (total dose of 500 mg/m2) gave the following peak plasma folate concentrations: dl-LV, 4.6 + 1.9 microM; 5-methyltetrahydrofolate, 4.3 + 2.1 microM; and no detectable l-LV in most cases. The d-LV/5-methyltetrahydrofolate ratios, however, were lower for the oral route than for the same dl-LV dose administered by 2-hour intravenous infusion in the same patient. To determine if there is a relationship between the dose, schedule, or route of administration and the therapeutic efficacy of LV combined with 5-FU, studies were carried out in rats with transplantable colon carcinoma. 5-Formyltetrahydrofolate was administered at various doses by either 2-hour infusion, 2-day continuous intravenous infusion, or by divided hourly oral doses for 4 hours. In all cases, the total doses of dl-LV administered were 100, 200, and 400 mg/kg. Data obtained to date indicate: (1) plasma folate concentrations by intravenous administration were dose dependent, but lower and saturable concentrations of folates were observed by oral administration; and (2) while the concentrations of 5-methyltetrahydrofolate achieved by the 2-hour infusion schedule were relatively constant and independent of the dose of dl-LV administered, conversion of dl-LV to 5-methyltetrahydrofolate with the 2-day infusion of 100, 200, and 400 mg/kg was dose dependent. In humans, however, conversion was independent of the route of administration of dl-LV (19% for the 2-hour infusion and 23% for the 5-day infusion of 500 mg/m2 dose). Preliminary results for antitumor activity of 5-FU in combination with dl-LV, administered by either 2-hour intravenous infusion (400 mg/kg/day for 5 days) or orally (100 mg/kg/hour for 4 hours), yielded similar inhibition of in vivo tumor growth, each being greater than what was achieved with 5-FU alone.(ABSTRACT TRUNCATED AT 400 WORDS)