Cyclic 3',5'-adenosine monophosphate (cAMP) and cyclic 3',5'-guanosine monophosphate (cGMP) are considered as potential biomarkers for Yin-Yang disharmony in traditional Chinese medicine.However,phosphodiesterase-mediated ex vivo degradation of these molecules in biological samples may result in their underestimation.In the present study,a ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) method was developed for determination of cAMP and cGMP in rat plasma,with special consideration of their stability ex vivo.Following precipitation of proteins from plasma samples with 0.4 M perchloric acid,the analytes were chromatographed on a Shimadzu Shim-pack-XR-ODS Ⅱ column with 2.5 mM ammonium acetate and methanol in gradient mode.The MS/MS detection was performed using multiple reaction monitoring in the positive electrospray ionization mode.The lower limit of quantification was 0.27 ng/mL for cAMP and 0.37 ng/mL for cGMP.The method was used to determine the plasma cAMP and cGMP levels in normal and Yin deficiency diabetic rats treated with or without Rehmannia glutinosa.The developed method may be useful for evaluating the regulatory effects of Chinese herbal medicine on the levels of cAMP and cGMP in the body.