Compounds for the treatment of elevated blood glucose levels and the physiological and metabolic disorders arising therefrom are disclosed which are inhibitors of the enzyme dipeptidyl peptidase IV (DDP-IV) and comprise novel substituted bicyclic 8-pyrrolidinoxanthines of formula I and their derivatives: wherein the various R-groups are defined herein. These compounds may be combined with other secondary active compounds which also are effective in the treatment of said disorders and the disease states or physiological manifestations arising therefrom.