A simple and sensitive column-switching HPLC method has been developed for the simultaneous determination of NK-104 (HMG-CoA reductase inhibitor) and its lactone in human and dog plasma. Plasma sample was extracted with methyl tert-butyl ether and then the extract was subjected to methylation with diazomethane to prevent the mutual conversion between NK-104 and its lactone. The extract was injected into the column-switching HPLC system. The calibration curves of NK-104 and NK-104 lactone were linear over the ranges 0.5 to 100 ng/ml for human plasma samples and 0.5 to 500 ng/ml for dog plasma, respectively. The intra-day and inter-day C.V. values of these analytes were less than 13.3%. The intra-day and inter-day accuracies of these analytes were between -14.0 and 6.5%. The proposed method has been applied to plasma samples obtained after oral administration of a single 2 mg dose of NK-104 to volunteers.