Abstract: The stereoselective chrono-pharmacokinetic parameters of hydratropic acid in rats were studied. The results showed that under standard light—dark cycle pharmacokinetic parameters of T_1/2α and CL are stereoselective and under reverse light—dark cycle, parameters T_1/2β, AUC, CL, Vc and MRT were shown to be stereoselective. To compare the corresponding parameters of the two different light—dark cycles using t—test, no differences were found in most of them.