Abstract: Kinetics of hydrolysis of flueytosine (5-FC) was discussed to predict the stability of flucytosine injections. Reverse phase ion-pair HPLC was used to determine 5-FC in flucytosine injections. The stability of the injections was examined with the classical isothermal method. The use of HPLC made it possible to determine the different contents of 5-FC in the presence of its degradation products. According to Arrhenius rule, the activation energy of 5-FC was 27.8 kcal. mol^-1. Since its main degradation product——fluorouracil is the factor in flucytosine clinical toxicity, shelf life was calculated in t_0.999 and t_0.998. At 25℃ it was about half a year (t_0.999) and 1.5 years (t_(0.998)). At 15～20℃ it was about 1.5 years (t_0.999) and about 3 years (t_(0.998)).