We propose to study the factors which control phospholipid metabolism in mammalian cells. We are studying two models in which phospholipid turnover can be experimentally manipulated. One model is surviving thyroid slices in which P32 incorporation into phosphotides can be stimulated incubating the slices with TSH. The other model is Ehrlich ascites cells in suspension culture in which calf serum is necessary for growth and in which the serum stimulates lipid turnover. In these systems the detailed biochemical steps of lipid synthesis are investigated under conditions of stimulation or rest to determine the control points in the biosynthetic sequences.