One if not the most important reason to study thy· roid hormone metabolism is the notion that most (in humans about 80%) of the overall 3, 3', 5· triiodo--thyronine (T3) production is generated in peripheral tissues by conversion of thyroxine (T4) into T3 (Chopra 1976). The liver plays an important role in this process but other tissues like kidney and brain are also of significance. The process of T4· deiodin· action into T3 is subject to regulation, eg in condi~ tions of caloric deprivation or starvation or in chro--nic or acute disease extra· thyroidal T3 production is diminished (Vagenakis 1981). It has been shown that the greater part ofT3 that occupies the nuclear receptor of rat liver, which is the main site for the initiation of thyroid honnone action, is not derived from local intracellular deiOdination of T4 but is derived from circulating plasma T 3 (Larsen 1982). In order to reach the nucleus, T3 has to be translocated from the extra· cellular fluid into the cell via the plasma membrane. The concentration of plasma T3 is, among other things, dependent on the amount of T4 which en· ters the tissue cells where it is converted into T 3• In the pituitary and especially in the cerebrum, most of the T3 bound to the nucleus is derived from intracellular deiodination of T4, less being derived from the plasma (Larsen 1982). If transport of T4 and T3 over the plasma membrane of target cells are controlled processes, then they may detennine (apart from processes which influence intracellular thyroid hormone metabo-lism) the ultimate production of T3 and the exertion of thyromimetic activity. Once, having entered tissue cells, thyroid hormones may be further metabolised or leave the cell unaltered. What has been mentioned with respect to the regulatory role of thyroid hormone influx may also apply to the efflux of these substances. In other words, if the efflux of thyroid hormones is subject to regulation then this process may also play a part in the control of thyroid hormone activity. We will discuss here studies concerning influx and efflux of thyroid hormones with special attention to possible regulation of these processes. Also the significance of these transport processes as studied by in vitro techniques will be discussed in the light of a possible physiological mechanism.