The "phospholipid effect" which is the enhanced turnover of the phosphorylinositol group of phosphatidylinositol (PI) occurs in the thyroid of response to thyreostimulin (TSH). The possibility that Ca2+ ions are involved in this stimulation has been investigated with pig thyroid slices. Experiments performed in media without Ca2+ or containing E.G.T.A. (2 mM), indicate that it is not the extracellular Ca2+ which is implied, but rather the intracellular Ca2+. The ionophore A23187 (6.10(-6) M) increases the specific radioactivity of the acid soluble precursors, but has also a specific effect on the PI turnover, which is additive with the effect of a high concentration of TSH (50 mU/ml). Washing and loading of slices with various Ca2+ concentrations show that 0.9 mM restores the TSH phospholipid effect. Verapamil (10(-3) M) and Chlorpromazine (10(-3) M) redirect glycerolipid metabolism by increasing PI and phosphatidic acid (PA) synthesis at the expense of other glycerolipids, as phosphatidylcholine (PC) and phosphatidylethanolamine (PE). These results suggest that the "phospholipid effect" is not a result of Ca2+ entry into the thyroid cells. On the contrary, it seems that this increased turnover of PI in "long term" incubations (3 hr). An additive and acute effect of TSH effect is more pronounced when Ca2+ movements