Eleven patients with stable rheumatoid disease (RD) who were receiving regular corticosteroid therapy (CS) were investigated to discover the effect on plasma prednisolone levels of additional therapy with the non-steroidal anti-inflammatory (NSAI) drugs, indomethacin and naproxen. There was a highly significant (P less than 0.001) increase in free prednisolone levels after concurrent therapy with either indomethacin or naproxen for 2 weeks. Total prednisolone levels were unchanged. These results could provide an explanation for clinical reports that these two NSAI drugs possess a steroid-sparing effect.