Résumé : In the rat thyroid cell lines PC Cl3, FRTL- 5 and WRT, proliferation is mainly regulated by insulin or IGF, and TSH. However, the mechanism regulating cell mass doubling prior to division is still unknown. Our laboratory has shown that in dog thyroid cells insulin promotes growth in size while TSH in the presence of insulin triggers DNA replication. In the absence of insulin, TSH has no effect on cell growth. In this report we investigated insulin action on both cell mass and DNA synthesis and its modulation by TSH and insulin in PC Cl3 and FRTL-5 cells. In PC Cl3 cells, insulin activated not only DNA synthesis but also protein synthesis and accumulation. Although TSH potentiated the stimulation of DNA synthesis induced by insulin, enhancement of protein synthesis by both agents was additive. All TSH effects were reproduced by forskolin. Similar effects were also obtained in FRTL-5 cells. This suggests that insulin and TSH, via cAMP, modulate both growth in size and DNA replication in these cell lines. Lovastatin, which blocks 3-hydroxy-3-methylglutaryl coenzyme A reductase, decreased the induction of DNA synthesis, but not of protein synthesis induced by insulin or TSH in PC Cl3 cells. In FRTL-5 cells, lovastatin reduced protein and DNA synthesis stimulated by insulin but not TSH-induced protein synthesis. Taking these data together, we propose that insulin and/or TSH both modulate cell mass doubling and DNA synthesis in these cell lines, presumably via different pathways, and that there are at least two pathways which regulate growth in size in FRTL-5 thyroid cells: one triggered by insulin, which is lovastatin sensitive, and the other activated by TSH, which is not sensitive to lovastatin.