Bioavailability of acetylated derivatives of digoxin tablets have been studied in healthy subjects after a single oral and intravenous dose as well as during maintenance therapy. alpha-acetyldigoxin shows a lower bioavailability than beta-acetyldigoxin even if the alpha-acetylated derivative is incorporated in a matrix of aerosil (SiO2). Moreover, beta-acetyldigoxin can be transferred to alpha-acetyldigoxin in alkaline solutions. This isomerisation leads to a decrease of the bioavailability of such fixed preparations which contain beta-acetyldigoxin and the hygroscopic salts of potassium-magnesium-aspartate. A prevention of the isomerisation is attained by isolating beta-acetyldigoxin from potassium-magnesium-aspartate. The bioavailability of a such new formulation is comparable to that of beta-acetyldigoxin alone. The experiments show the bioavailability of acetylated derivatives of digoxin to be influenced by the physico-chemical properties of a drug and its preparation.