Fifteen patients (30 to 78 years of age) with diagnoses of rheumatoid arthritis were administered oral and intravenous methotrexate (15 mg), alone or with concomitant naproxen (1000 mg/day). Serial blood samples and urine were collected for 24 hours after the dose of methotrexate and were assayed for methotrexate by a specific radioenzymatic method. In twelve patients who completed the study, methotrexate systemic clearance was not statistically different with naproxen (103.3 +/- 35.0 ml/min) versus without naproxen (113.4 +/- 48.3 ml/min; p = 0.37). Oral clearance of methotrexate was not statistically different with naproxen (161.7 +/- 55.0 ml/min) versus without naproxen (176.7 +/- 68.3 ml/min; p = 0.14). Likewise, there was not a significant difference in methotrexate renal clearance or plasma protein binding with or without naproxen. No toxicity was observed when patients received methotrexate alone or with naproxen. This study indicates that concomitant naproxen does not abruptly alter the disposition of low-dose methotrexate in patients with rheumatoid arthritis who have normal renal function.