Six healthy men (22-31 y) were supplemented for 4 wk with folic acid labeled with deuterium [3',5'-2H2; 3.6 mumol/d (1.6 mg/d)] to permit evaluation of in vivo kinetics of this vitamin. Total folate in urine, serum and erythrocytes was determined by microbiological assay, and isotopic labeling of urinary and erythrocyte folate was determined by gas chromatography-mass spectrometry. During supplementation, serum folate reached maximal concentration in approximately 18 d, whereas excretion of total and deuterium-labeled folates increased rapidly and reached isotopic steady state in 1-2 wk. Isotopic labeling of erythrocyte folate increased continually over the entire supplementation period. Upon cessation of supplementation, red blood cell folate and urinary folate excretion (total and labeled) decreased linearly. The decline in total serum folate could be described with a biexponential model that yielded a slow-phase half-life of 18.7 +/- 2.3 d. This model also indicated a turnover of 4.5% of the total body folate pool per day. Pool sizes of total body folate before and after supplementation (at steady state) were calculated to be 10 mumol (4.4 mg) and 98.9 mumol (43.7 mg), respectively. These kinetic data and stable isotope methodology may be used to address a wide range of experimental questions related to folate metabolism.