A series of clinical pharmacology studies was performed to screen for possible pharmacokinetic/dynamic contributions to drug interactions reported in rheumatoid arthritis patients receiving cyclosporine and nonsteroidal antiinflammatory drugs (NSAIDs). No clinically relevant pharmacokinetic changes in any of the drugs were noted when single-dose cyclosporine was coadministered during a steady-state regimen of aspirin, indomethacin, or piroxicam in healthy volunteers. Only with diclofenac was an interaction observed whereby the diclofenac area under the concentration-time curve was doubled in the presence of cyclosporine. Based on the outcomes of the screening studies, steady-state coadministration of both diclofenac and cyclosporine was assessed in rheumatoid arthritis patients, confirming the drug interaction of diclofenac with cyclosporine. Although the drug interaction was accompanied by a significant rise in serum creatinine, there was no apparent concentration-effect relationship, inasmuch as the increase in diclofenac exposure was not related to the magnitude of increase in serum creatinine. Based on the results of these five drug-drug interaction studies and the known biotransformation pathways of nonsteroidal antiinflammatory drugs, it is speculated that the pharmacokinetic interaction, which is unique to diclofenac, is caused by inhibition by cyclosporine of diclofenac's first-pass metabolism. Caution and appropriate clinical monitoring are recommended whenever cyclosporine and NSAIDs are coadministered; however, diclofenac in particular should be administered near the lower end of its therapeutic range when it is initially combined with cyclosporine in the treatment of rheumatoid arthritis.