Selenium has long been recognized as a dietary antioxidant and is now known to be an essential component of the active sites of several enzymes, including glutathione peroxidase [2] and thioredoxin reductase, which catalyze reactions essential for the protection of cellular components against oxidative and radical damage [3]. Moreover, previous work in our laboratory describes the synthesis of pyrimidoselenolo- [2,3-b]quinoline [4] and pyrimidoselenolo[2,3-c]pyridazine derivatives [5] and indicates that certain compounds possess significant anti-inflammatory and analgesic activities with strong fungicidal effects. Furthermore, in Part 1 [6] and Part 2 [1] we published the synthesis of selenolo[2,3-b]pyridine, pyrido[3',2':4,5]selenolo- [3,2-d]pyrimidine, 2,4-dimethyl-7,8-dihydropyrrolo[1,2-a]pyrido[3',2':4,5]selenolo[3,2-d]pyrimidin-10(6H)-one, 7,9-dimethylpyrido[3',2':4,5]selenolo[3,2-e][1,2,4]triazolo[4,3-c]pyrimidine, 7,9-dimethylpyrido[3',2':4,5]selenolo[ 3,2-d]pyrimidine-2,4(1H,3H)-dithione, 7,9-dimethylpyrido[3',2':4,5]selenolo[2,3-e]tetrazolo[1,5-c]pyrimidine- 6(5H)thione, 9,11-dimethylpyrido[3',2':4,5]selenolo[2,3-e][1,2,4]triazolo[1,5-c]pyrimidine, 7,9-dimethylpyrido[ 3',2':4,5]selenolo[2,3-e]imidazo[1,2-c]pyrimidine, and 10,12-dimethylpyrido[3'',2'':4',5']selenolo- [3',2':4,5]pyrimido[1,6-a]pyrimidine derivatives. Prompted by these observations and in continuation of our work we report herein new classes of fused tri- and tetracyclic systems containing the selenolo[2,3-b]pyridine fragment in the hope that members of these series may find interesting biological applications.