We investigated the uptake of L-tri-iodothyronine (T3) by cultured human trophoblast cells. Uptake was time-dependent, initially linear and approaching equilibrium after 60 min with an approximate half-time of 13 +/- 4.5 min (mean +/- S.E.M., n = 4). It had a non-saturable component accounting for about 50% of total uptake. We demonstrated a single saturable T3 uptake mechanism with a calculated Michaelis constant (Km) of 755 +/- 145 nmol/l (n = 11-13) and a corresponding maximum velocity of 28.8 +/- 5.3 pmol/min per mg protein (n = 11-13). The Km value was similar to those reported in other tissues.