The thyroid hormone 3,3',5-triiodothyronine (T3) plays an important role in the differentiation of adipocytes, as well as in the thermogenic activity of brown adipose tissue. Recently a T3 binding protein (T3BP), which is associated with plasma membranes, has been isolated and cloned from liver. It proved to be homologous to the multifunctional enzyme protein disulfide isomerase (PDI), which is involved in posttranslational modifications of secretory proteins. In this study we investigated the T3BP/PDI mRNA expression in white and brown adipose tissue of rat and bovine, as well as in several rodent adipose cell lines at various states of differentiation. T3BP/PDI mRNA expression was found in white and brown adipose tissue, as well as in preadipocytes and adipocytes at all states of differentiation. Comparison to other tissues (liver, kidney, heart, brain) revealed that its expression was highest in white fat. No modulation of T3BP/PDI mRNA corresponding to different adaptational or developmental situations could be detected in adipose tissue.