This study looks at how the arthritis drug methotrexate works in low doses to treat rheumatoid arthritis. (High doses of methotrexate are used to treat some types of cancer.) Methotrexate blocks the action of the B-vitamin known as folic acid. We are studying the biochemical reactions affected by this vitamin because we think that blocking many of these reactions may be necessary for methotrexate to work in treating rheumatoid arthritis. Through these studies, we hope to gain a better understanding of how this drug and related drugs work as treatments for arthritis.