Propranolol is a widely used Beta-blocking agent which has been found useful in the treatment of hypertension. We are developing a sensitive and specific assay for propranolol to assess the specificity of existing procedures and extend the assay to include several metabolites of the drug. To accomplish this we are synthesizing deuterated analogues of propranolol and its metabolites for use as stable isotope internal standards for gas chromatography-mass spectrometry. Based on our initial results with plasma extracts of propranolol and propranolol d5, selected ion monitoring will be necessary for a sensitive and reproducable assay.