OBJECTIVE The stability of pravastatin under simulated in vivo conditions was studied.METHODS Residule concentrations of pravastatin and concentrations of 3'α-isopravastatin in different pH solutions were determined by HPLC,which deduced the transformation kinetics functions.RESULTS The results showed that pravastatin was unstable under the silnulated stomach conditions it transformed rapidly especially in gastric juice under fasted conditions.Initial transformation of pravastatin displayed first-order kinetics.Pravastatin was mainly transformed to 3'α-isopravastatin.The optimal pH value for the transformation was 2.Pravastatin was stable under the conditions of simulated the intestine pH and no transformation was found.CONCLUSION Attention should be paid to the transformation of pravastatin to 3'α-isopravastatin under aqueous acid conditions.Therefore several formulation strategies should be applied in order to reduce the transformation of pravastatin in the stomach and improve its bioavailability.