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A two-step release system (TSRS) for the compound Danshen, which has drug-release behavior that is in accordance with the circadian rhythms of cardiovascular disease, was developed by combining an effervescent osmotic pump tablet and a pulsed-released tablet into one hard capsule by our lab. Anin vivostudy indicated that after oral administration of TSRS, two peaks of the plasma concentration of both Danshensu (DS) and protocatechuic aldehyde (PA) were observed, which suggested that the drug plasma concentration–time curve could meet the requirements for chronotherapy of cardiovascular disease after the bed-time administration of such a device. High performance liquid chromatography using an ultraviolet (UV) detector was used to simultaneously determine the concentrations of DS and PA in plasma. This method was simple, convenient, and appropriate for the quality control of DS and PA. A linear correlation model was established based on the percent absorbant data and percentin vitrodissolution data. Because the drugs were released from the device in an osmotic pressure-dependent manner and absorbed rapidly, a reasonable linear regression relationship was observed between thein vitroandin vivoperformances. The current study highlights the potential use of such a device for chronopharmaceutical drug delivery. [ABSTRACT FROM AUTHOR]