To investigate the effects of methotrexate (MTX) on the pharmacokinetics (PK) of CP-690,550, a novel Janus kinase (JAK) inhibitor in development as a therapy for rheumatoid arthritis (RA), to determine the effects of multiple doses of CP-690,550 on the PK of MTX, and to evaluate the short-term safety and tolerability of co-administration of CP-690,550 and MTX.