1. The pharmacokinetics of MTX and its 7-hydroxy metabolite (7-OHMTX) were investigated in nine patients with rheumatoid arthritis (RA). Each patient received 15 mg MTX i.v., i.m. and p.o. after an overnight fast in a randomized cross-over design. The plasma concentrations of MTX and 7-OHMTX were measured over 7 days and their urinary excretion over 24 h. 2. Plasma concentrations of MTX were described by a triexponential function after i.v. administration, a triexponential function with zero or first order absorption after oral administration, and a biexponential function with zero of first order absorption after i.m. injection. Plasma concentrations of 7-OHMTX were described by a biexponential function after all three routes of administration. The median terminal elimination half-lives of MTX and 7-OHMTX were 55 h and 116 h, respectively. The area under the plasma concentration-time curve (AUC (0,170 h)) of MTX did not differ between i.m. and oral administration indicating similar bioavailability after these routes of administration. The AUC (0,170 h) values of 7-OHMTX after i.v., oral and i.m. administration were similar. Over 80% of MTX was excreted in urine as intact drug and about 3% was excreted as 7-OHMTX during 24 h after drug administration. 3. Plasma concentrations of MTX and 7-OHMTX were measurable at the end of the dose interval in most of the patients and may help to identify non-responders or patients with increased risk of side-effects.