Abstract: The in vitro release of isosorbide dinitrate from a sustained-release tablet was determined by using 0.1N HCl, a simulated gastric fluid for the first 3 hr and phosphate buffer solution pH 7.2～7.4, an simulated intestinal fluid for the subsequent 3 hr. The drug was gradually released and was completely dissolved at the end of 6 hr. When the tablet was introduced directly into the stomach of anesthetized cats, a 63% fall in arterial blood pressure was observed which persisted for more than 10 hr. A good correlation between the rate of the percentage release of the drug in vitro and the in vivo effect in sustained percentage lowering of the blood pressure was obtained. The in vitro method described is proposed for the quality and bioavailability control of isosorbide dinitrate sustained-release tablets.