Paeoniae Radix (PR) is a commonly used traditional Chinese medicine. A slight effect of PR on the pharmacokinetics of phenytoin that is mainly metabolised by CYP2C9 has been reported. The aim of this pilot study was to clarify if PR has an effect on losartan oxidation used as a measure of CYP2C9 activity. Three healthy volunteers received a single oral dose of losartan before and after PR treatment. Losartan and E-3174, an active metabolite of losartan, were analysed in 8 hour urine. PR did not seem to have an effect on CYP2C9 activity when the losartan/E-3174 ratios were compared before and after PR treatment (P = 0.56) although a larger study would need to be undertaken to confirm this finding.