In a double blind placebo controlled crossover study, each arm of 4 weeks' duration, 20 mg of folinic acid/week or placebo were administered to 13 patients with rheumatoid arthritis. These individuals were receiving weekly intramuscular methotrexate (MTX) but were about to discontinue because of side effects. While there was considerable improvement in nausea during the study, the effect of folinic acid could not be differentiated from that of placebo. There was no adverse effect on control of disease activity. It therefore seems likely that polyglutamated tissue stores of MTX do not contribute to drug efficacy and in this format folinic acid could not be shown to be more useful than placebo in reducing drug induced nausea.