The clinical effects and pharmacokinetics of high-dose uridine were determined in seven patients with advanced-stage cancer and in one healthy volunteer. Uridine was also examined for its effect on 5-fluorouracil toxicity in two patients. Uridine was administered as a 1-hr i.v. infusion at doses of 1 to 12 g/sq m. Plasma and urine samples were analyzed for uridine and uracil using high-pressure liquid chromatography. In 23 courses of uridine alone, the only toxicity observed was transient shivering after one of two courses at 12 g/sq m. This side effect was also seen after administration of uridine (10 g/sq m) during combination with 5-fluorouracil. The pretreatment plasma uridine concentration was elevated from low micromolar to millimolar levels with uridine administration at doses up to 12 g/sq m. Maximal areas under the concentration-time curve were about 5 mmol/liter/hr. Both peak plasma level and area under the curve for uridine increased linearly with dose. Uridine plasma decay curves were biphasic with a terminal half-life of 118 min. Half-life, volume of distribution (634 ml/kg), and total clearance (4.98 ml/kg/min) appeared to be independent of dose. Plasma uracil concentration increased gradually after administration of uridine to plateau levels. Maximal plasma uracil concentrations were about one-tenth that of peak uridine concentrations. The plasma uracil level declined with a half-life of about 40 min after uridine levels decreased to 300 microM. Total urinary excretion of uridine was 24% of the dose, while the amount of uracil recovered in urine was 3.4%. In two patients, uridine rescue was attempted during 5-fluorouracil dose escalation. Uridine at 5 to 6 g/sq m given on 1 or on 2 days after 5-fluorouracil did not prevent myelosuppression and gastrointestinal toxicity associated with increasing plasma concentrations of 5-fluorouracil. These data show that uridine administered as a 1-hr infusion at doses which provide peak plasma uridine concentrations in the millimolar range is well tolerated. Rapid elimination of uridine primarily due to catabolism results in modest exposure to substantially elevated plasma uridine concentrations. Preliminary findings suggest that prolonged treatment with uridine may be required to test its potential to rescue patients from 5-fluorouracil toxicity.