Because the 5'-flanking hypervariable region of the human insulin gene may be associated with insulin-dependent diabetes mellitus (IDDM), we examined the spontaneously diabetic BB rat and other rat strains for polymorphisms of the two rat insulin genes, the localization of such polymorphisms, and their possible association with IDDM. By use of restriction-fragment-length polymorphism analysis, we found that the transcribed portion of the insulin I gene, its 3'-flanking region, and the insulin II gene were not polymorphic. However, four alleles of the insulin I gene were identified, two of which (IA and IB) were found in BB rats. Alleles IA and IB varied in their 5'-flanking regions, yet neither was associated with IDDM in the BB rat.