Plasma and urinary concentrations of (+/-)-(R*)-2,6-dimethyl-4-(m-nitrophenyl)-1,4-dihydropyridine-3,5-dicarb oxylic acid (R*)-1-benzyl-3-piperidinyl ester, methyl ester hydrochloride (benidipine hydrochloride, KW-3049) were measured by the radioreceptor assay using 3H-nitrendipine and rat cardiac membrane receptor. After methanol deproteinization and ethyl ether extraction under alkaline conditions as pretreatment, the detection limit for KW-3049 in plasma was 0.2 ng/ml using 0.5 ml of sample. For the urine, 0.05 ml of urine sample was directly added to the assay system, and detection limit of drug was 1 ng/ml. The cross-reactivity of the presumable metabolites was 1% or less of the unchanged drug. This radioreceptor technique was applied to pharmacokinetic studies and useful for monitoring the drug after therapeutic dosing.