The invention relates to the use of N,N'-dihydroxyamidine (I), N,N'-dihydroxyamidine ether (II), N,N'-dihydroxyamidine diether (III), N,N'-dihydroxyamidine ester (IV), N,N'-dihydroxyamidine diester (V) or 4-hydroxy-1,2,4-oxadiazoline (VI) of the formulae cited above, wherein R represents hydrogen, an alkyl and/or aryl radical, as a substitute for an amidine function of a medicament for improving the bioavailability of the medicament.