Transthyretin (TTR) cDNA probes were used to determine the presence of TTR mRNA in Northern blots from rat, porcine, and human organs as well as from human endocrine tumors. We also used in vitro translation in our study of the human tissues. In accordance with previous findings, TTR mRNA was found in the choroid plexus and, to a lesser extent, in the liver of all three species. In addition low levels of TTR mRNA were identified in the rat and human pancreas. All of the endocrine pancreatic tumors (two glucagonomas, two insulinomas, and one nonfunctional tumor) and the gut carcinoid also contained TTR mRNA, whereas other endocrine tumors (two pheochromocytomas and one paraganglioma) and one adenocarcinoma of the pancreas were TTR mRNA negative. The level of TTR mRNA expression in most of the endocrine pancreatic tumors exceeded that in the liver. The in vitro translations produced pre-TTR molecules of similar size for all TTR mRNA-positive human organs and tumors.