1. The metabolism of phosphatidylinositol in pig thyroid has been investigated as a basis for understanding the specific stimulation of the synthesis of this phospholipid in the gland by thyrotropin. 2. The gland contained an active Ca 2+ -dependent phosphatidylinositol-splitting enzyme with an optimum pH of 5.3–5.5. 3. The major water-soluble product (65%) formed by this catabolic enzyme was not phosphorylinositol but a related compound, which may be a cyclic phosphorylinositol. Both this and phosphorylinositol (35%) were released simultaneously from the phosphatidylinositol substrate. 4. The phosphatidylinositol-splitting enzyme was found almost exclusively in the supernatant fraction obtained by homogenization of the gland. It was not present in the acid-phosphatase-containing particulate fraction. 5. The incorporation of [2- 3 H 1 ]inositol into phosphatidylinositol in the presence of either CDP-diglyceride or CTP+ATP was most active in the microsomal fraction. 6. When thyroidal microsomes were labelled with [ 3 H]inositol and 32 P, and then incubated with unlabelled inositol, there was a dramatic loss of 3 H labelling from the phosphatidylinositol, which was not accompanied by an equivalent loss of 32 P from the phosphate moiety. This turnover of the inositol moiety required nucleotide coenzymes. It is postulated that the phosphatidylinositol is split into inositol and a phosphorus-containing lipid precursor of the phospholipid that remains on the microsomal membrane and is recycled. 7. Isolated thyroidal mitochondria synthesized phosphatidylinositol from [2- 3 H 1 ]inositol only because of their contaminating microsomal component. 8. Some evidence has been obtained of a rapid transfer of phosphatidylinositol molecules from thyroidal microsomes to mitochondria when these were incubated together in the presence of a supernatant fraction. 9. Both phosphatidylinositol breakdown by the supernatant fraction of the gland and synthesis by the microsomes were totally inhibited by 1m m -chlorpromazine. This drug is known to suppress thyrotrophin-induced stimulation of activity in thyroid slices.

1. The effects of thyrotrophin in vitro on the incorporation of [ 14 C]-glucose, -glycerol, -palmitate and -oleate into the lipids of thyroid tissue were examined. 2. Thyrotrophin increased the incorporation of these 14 C-labelled precursors into phosphatidylinositol specifically. 3. Thyrotrophin also increased the proportion of 14 C radioactivity from labelled glucose, glycerol, palmitate and oleate incorporated into the 1,2-diglycerides. 4. The addition of thyrotrophin to thyroid slices for 10min., after 2hr. of prelabelling with [ 14 C]glycerol, also increased the proportion of 14 C radioactivity incorporated into the 1,2-diglyceride fraction. 5. After incubation of thyroid tissue with [1− 14 C]palmitate, thyrotrophin caused a two- to three-fold increase in the specific radioactivity of palmitate isolated from phosphatidylinositol and 1,2-diglycerides. In contrast, the specific radioactivity of palmitate isolated from the choline and ethanolamine phosphoglycerides, 1,3-diglycerides and triglycerides was not increased by thyrotrophin.