Insulin-degrading enzyme (IDE), a cytosolic metalloendoprotease, can degrade insulin, insulin-like growth factor-II, insulin-like growth factor-I, and transforming growth factor-alpha. While IDE has been implicated in the cellular degradation of insulin, other physiological functions of this enzyme are not known. To assess the possible role of IDE in cellular growth and development, we determined the tissue and developmental distribution of the enzyme. Rat IDE cDNA fragments and antibodies directed against human IDE were used to probe IDE transcripts and proteins in rat tissues. The results demonstrate that IDE transcripts are ubiquitous in rat tissues. The level of rIDE transcripts is high in adult rat testis, tongue, and brain; moderate in kidney, prostate, heart, muscle, liver, intestine, and skin; and low in spleen, lung, thymus, and uterus. The sizes of the major transcripts of rIDE are 3.4 and 6.3 kilobases in all tissues analyzed, except testis. Surprisingly, the highest level of rIDE mRNA in the adult rat was in the testis, and the major transcripts of rIDE in this tissue were shifted in size to 3.8 and 6.7 kilobases. Immunocytochemical analysis localized the rIDE mainly in the epithelium of prostate gland and kidney, and the cytosol of liver hepatocytes. During rat development from 6-7 days of age to adulthood, rIDE mRNA levels increased in brain, testis, and tongue; decreased in muscle and skin; and did not significantly change in other tissues examined. These studies reveal regulation of IDE or IDE-related genes in rat tissues and during rat development, suggesting that this enzyme may have multiple functions relating to cellular growth and development.