Previous reports showed pulsatile secretion of TSH in man. Therefore, we investigated the effect of discontinuous versus continuous TSH stimulation on the cellular level of FRTL-5 thyroid cells, namely the expression of a nuclear proliferation antigen (NPAg). The expression of this antigen correlates linearly with the 3H-thymidine incorporation and is a marker for cellular growth. The FRTL-5 cells were stimulated for 4 hours with bTSH (0.01-1.0 U/l). Compared to continuously stimulated cell cultures the discontinuous stimulation of FRTL-5 cells with bTSH showed a significant higher rate of NPAg expression, i.e. cellular growth.