In FRTL-5 cells, the osmolyte taurine is released both during swelling and thyrotropin (TSH) stimulation. To try to identify the signaling systems engaged during these releases the [3H]taurine efflux rate coefficient was monitored. Cells grown at standard conditions in culture dishes (35 mm Ø) were loaded with [3H]taurine and adapted to flow-through chambers (35 x 0.5 mm, 0.5 ml/min). Fractional release was monitored during one min periods. 12% reduction in osmolality or 10 mU/ml TSH in iso-osmotic medium increased the efflux rate coefficient to about the same peak level. The phosphodiesterase inhibitor 3-isobutyl-1-methyl xanthine (100 mM) increased these peak levels 6.4 and 2.6 times, respectively. Dibutyryl-cAMP (500 mM) mimicked the swelling and TSH activated taurine efflux. The phospholipase A2 activator melittin (1 mg/ml) increased the efflux activity 324 ± 60% above control level (100 ± 15%). The 5-lipoxygenase inhibitor ETH 615-139 (8 mM) inhibited the osmolality activated taurine efflux peak rate coefficient 60.4 ± 4%. Cholera toxin inhibited the TSH effect, but increased the swelling effect 191 ± 35%. It is already known that the TSH receptor (TSHR) is Na+ sensitive. Inhibition the Na-H exchanger (5-(N,N-dimethyl)amiloride, 100 mM) increased the TSH-mediated efflux activity 204 ± 20%; ouabain (1 mM) inhibited the same 51.2 ± 8.5%. Na-free medium increased the TSH activation by 540 ± 51% and the swelling activation by 143 ± 7%. Based on the present results and earlier observations, I suggest that the adenylyl cyclase–cAMP pathway and phospholipase C–arachidonic acid pathway might mediate the taurine efflux activation. The TSHR appears to activate both these pathways induced by TSH and swelling.