Considerable interest has recently been focused on the putative role of mutations in the mitochondrial genome for the development of noninsulin-dependent diabetes. The Goto-Kakizaki (GK) rat, a genetic model of defective insulin secretion and hyperglycemia, is characterized by partial maternal inheritance. Because the mitochondrial genome is known to be maternally transmitted, the aim of this study was to investigate whether the GK syndrome can be explained in terms of alterations of the mitochondrial DNA (mtDNA). For this purpose, pancreatic islets were isolated from adult and fetal control Wistar and diabetic GK rats. Using electron microscopy, the ultrastructural morphology of beta-cell mitochondria was analyzed in control and GK islets. It was found that the beta-cells of adult GK rats had a significantly smaller mitochondrial volume and an increased number of mitochondria per unit tissue volume as compared with the beta-cells of corresponding control islets. Moreover, mtDNA and mtRNA were isolated from the islets and, as a control tissue, from liver, and subsequently analyzed using Southern and Northern blot techniques. No major deletions or restriction fragment polymorphism could be detected in mtDNA from both GK liver and GK islets. The mtDNA sequence of the transfer RNAleu(UUS) gene was identical in both strains of rats. mtDNA contents of fetal GK islets and fetal GK liver were not different from those of fetal Wistar rats. However, adult GK islets contained markedly less mtDNA than the corresponding control islets, contrary to the mtDNA contents of adult liver, which were similar in the two strains. The lower islet mtDNA contents were paralleled by a decreased content of islet mtRNA (12S ribosomal RNA and cytochrome b messenger RNA). Islet insulin messenger RNA contents were similar in GK and Wistar rats. In conclusion, our results do not support a role of a genetic defect in mtDNA as a cause of the GK syndrome. Instead, mtDNA damage may occur specifically in islet cells as a consequence of the disturbed metabolic environment of the adult GK rat. It is speculated that a long-lasting metabolic dysfunction may induce mtDNA damage and/or inhibition of mtDNA replication leading to a gradual and late decrease in the mitochondrial volume fraction and subsequently an impaired capacity for oxidative metabolism.