Functions of taurine in animal cells have caught interest for a long time. Its efflux activity has been well-studied during cellular volume regulation, but has also been investigated during stimulation with hormones. In FRTL-5 rat thyroid cells, both cellular swelling and TSH stimulation have been shown to activate taurine efflux. Little is known, however, about taurine efflux in polarized cells, and it was of interest to localize this activity with respect to the apical and the basolateral membrane. FRTL-5 cells were grown in culture dishes with permeable membranes as bottoms, to establish polarized cell-layers. The cells, preloaded with [3H]taurine, were installed in a perfusion chamber, where two chamber-parts, apical and basolateral, were separated by the cell-layer. Media was perfused through the chamber at a constant flow, and collected separately for counting. Thus the experiments enabled to distinguish between apical and basolateral taurine efflux. The cells demonstrated polarity in this activity, most pronounced during TSH application on the basolateral side, which lead to a significant activity increase only on the apical side. Hyposmotic exposure stimulated activity on both sides, but was also polarized, as the rate coefficient for apical efflux was two times higher than the basolateral. TSH applied on the apical side induced taurine efflux on both sides, where the difference between the apical and the basolateral activity was not statistically significant. Arachidonic acid metabolites seemed to participate in the swelling-activated efflux, whereas cAMP was not involved. TSH appeared to activate both cAMP-dependent and cAMP-independent taurine efflux. The results indicate that there may be several different types of taurine efflux pathways in polarized thyroid cells. It is also possible that there is more than one type of channel mediating efflux of taurine.