The present invention relates to 2-amino-imidazo[4,5-d]pyridazin-4-ones and 2-amino-imidazo[4,5-c]pyridin-4-ones of general formula ##STR00001## wherein R.sup.1 to R.sup.4 and X are defined as in claims 1 to 6, the tautomers, the enantiomers, the diastereomers, the mixtures thereof and the salts thereof, which have valuable pharmacological properties, particularly an inhibiting effect on the activity of the enzyme dipeptidylpeptidase-IV (DPP-IV).