On the basis of the very interesting pharmacological properties shown by the 5-amino[1,2,4]triazolo[4,3-a][1,8]naphthyridine-6-carboxamide derivatives 1, previously described by us, we have now prepared the 5-aminoimidazo[1,2-a][1,8]naphthyridine-6-carboxamide derivatives 2a-o (a new structural class) whose tricyclic system is isosteric to that of compounds 1. Both compounds 2 and some new properly substituted compounds 1 (1f-k) now synthesized were tested in vivo for their analgesic and anti-inflammatory activities: on the whole, compounds 2 showed notable analgesic properties, whereas many compounds 1 exhibited a very potent anti-inflammatory activity, coupled to scarce analgesic activity. All the effective compounds proved to be completely devoid of acute gastrolesivity (gastric damage) in rats (at the 200 mg kg(-1) oral dose).