Eleven patients treated with a 5.5-day continuous i.v. infusion of 500 mg/m2/day of (6R,S)-folinic acid in combination with daily bolus 5-fluorouracil had a median steady-state plasma concentration of 3.25 microM (6S)-folinic acid (the bioactive diastereoisomer). The bioactive metabolite (6S)-5-methyltetrahydrofolic acid, analyzed in six patients, reached a median steady-state plasma concentration of 5.7 microM. The lowest plasma concentrations at steady-state were 1.86 microM (6S)-folinic acid and 3.12 microM (6S)-5-methyltetrahydrofolic acid. These concentrations are above the minimum concentrations shown by other investigators to produce synergism between (6R,S)-folinic acid and 5-fluorouracil in vitro. The median steady-state plasma concentration of (6R)-folinic acid was 38.2 microM, more than 10 times the concentration of (6S)-folinic acid. Along with other plasma pharmacokinetic parameters, terminal half-lives were estimated for (6S)-folinic acid (median, 45.4 min), (6R)-folinic acid (median, 388 min), and (6S)-5-methyltetrahydrofolic acid (median, 446 min). Investigation of the renal pharmacokinetics confirmed the marked difference in the renal clearance of the two diastereoisomers of folinic acid which had been observed after low doses of (6R,S)-folinic acid (J. A. Straw, D. Szapary, and W. T. Wynn, Cancer Res., 44: 3114-3119, 1984). However, the low renal clearance of (6R)-folinic acid (median, 8.2 ml/min/m2) was attributable to the extensive binding of (6R)-folinic acid to plasma proteins (median, 8.7% free), not to reabsorption in the kidney.