['To evaluate the effects of repeated doses of folic acid on the pharmacokinetics of methotrexate (MTX) in patients with rheumatoid arthritis.', 'We studied 20 patients (ages 30-78 years) who received MTX intramuscularly (10 mm/week). MTX was administered alone or after treatment with folic acid (5 mg tablet once daily) for 13 days. Plasma samples were collected 2 and 8 h after dose intake. MTX concentrations in plasma and ultrafiltrate samples were measured by fluorescence polarization immunoassay. A Bayesian approach was used to determine individual MTX pharmacokinetic variables to minimize the number of samples collected.', 'Folic acid supplementation led to reduced plasma MTX levels 2 and 8 h after MTX administration and reduced area under the plasma MTX concentration versus time curve (AUC) (about 20%; p < 0.02). Total clearance of MTX and Vd were higher when patients were also receiving folic acid than when they were taking MTX alone (p < 0.02). The plasma protein binding of MTX remains unchanged.'] Does folic acid alter methotrexate availability in patients with rheumatoid arthritis?