Novel heterocyclic dihydropyrimidine compounds useful as inhibitors of potassium channel function (especially inhibitors of the K.sub.v1 subfamily of voltage gated K.sup.+ channels, especially inhibitors K.sub.v1.5 which has been linked to the ultra-rapidly activating delayed rectifier, which have the structure stereoisomers including enantiomers thereof and diastereomers thereof, or a pharmaceutically acceptable salt thereof, wherein Q is alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl, cycloheteroalkyl, substituted cycloheteroalkyl or which is aryl, substituted aryl, heteroaryl or substituted heteroaryl; R.sup.3 is wherein R.sup.1, R.sup.2, R.sup.4, R.sup.5, R.sup.6, R.sup.7, R.sup.8, R.sup.9 and R.sup.10 are defined herein. Methods of using such compounds in the prevention and treatment of arrhythmia and I.sub.Kur-associated conditions, and pharmaceutical compositions containing such compounds are also provided.