A convincing line of evidence is being developed that the congenital nongoitrous hypothyroidism and dwarfism observed in the WIC-rdw rat may indeed be caused by a primary defect in thyroid hormonogenesis. In support of this hypothesis, several recent reports have shown the presence of elevated molecular chaperone levels in the WIC-rdw thyrocytes, the endoplasmic reticulum of which was markedly dilated, suggesting a defect in intracellular protein transport. Here the studies were undertaken to identify the precise molecular defect in the WIC-rdw rat. First, the genetic linkage analysis revealed that the rdw locus was on rat chromosome 7 and was identical to the thyroglobulin (Tg) gene locus. Moreover, the Tg protein level was reduced in the WIC-rdw thyroid despite a similar level of the Tg gene transcripts that were indistinguishable in their size from the normal. Next, the complete sequencing of the rdw and the normal rat Tg cDNAs revealed a single nucleotide change, G6958C, resulting in a G2320R missense mutation in a highly conserved region of the Tg molecule. Finally, transient expression of the intact Tg cDNA containing the rdw mutation in the COS-7 cells showed no detectable Tg in the secreted media, indicating a severe defect in the export of the mutant Tg. Together, our observations suggest that a missense mutation, G2320R, in the Tg gene is responsible for the rdw mutation in the WIC-rdw rat.