The pharmacokinetic profile of total and free methotrexate (MTX) and the effect of piroxicam on MTX pharmacokinetics was studied in 20 rheumatoid arthritis patients receiving a stable dosage of MTX (10 mg/week). Plasma protein binding ranged from 25 to 55%. To describe the variations with time of the unbound fractions a mathematical characterization relationship between the total and free MTX was used. Total and free MTX were correlated with the sigmoid maximum effect model. The slope factor (gamma) was proportional to the number of binding sites. The free fraction for a given patient can be evaluated from this relationship. Total clearance of MTX was not statistically different with piroxicam (8.0 l/h for total MTX, 13.7 l/h for free MTX) vs without piroxicam. Likewise, there were no significant difference in tmax, area under the plasma concentration vs time curve, distribution and elimination half-lives, mean resonance time, and volumes of distribution. Although the highest observed total MTX concentration was significantly lower with piroxicam, there were no significant pharmacokinetic interactions between low-dose MTX and piroxicam.