1. The phospholipid composition of fat-cells prepared from rat epididymal fat-pad was determined. 2. The incorporation of [(32)P]P(i) into the phospholipids of fat-cells incubated in glucose-free medium and the effect of adrenaline and of alpha- and beta-adrenergic blocking agents, were studied. 3. Incorporation of [(32)P]P(i) into fat-cell phospholipid increased with time; incubation with adrenaline resulted in increased incorporation that was related to the concentration of adrenaline. 4. The pattern of incorporation of [(32)P]P(i) into the individual phospholipids of fat-cells after incubation for 1h was determined; adrenaline (5.4mum) resulted in increased incorporation into phosphatidylcholine. 5. Incubation of fat-cells with propranolol (34mum) and adrenaline (5.4mum) resulted in abolition of adrenaline-stimulated lipolysis; there was a decrease in the specific radioactivity of phosphatidylcholine and an increase in the specific radioactivity of phosphatidylethanolamine, phosphatidic acid, phosphatidylinositol and cardiolipin compared with cells incubated with adrenaline alone. 6. Incubation of fat-cells with phenoxybenzamine (0.1mm) and adrenaline (5.4mum) resulted in stimulation of lipolysis, and in diminished specific radioactivities of phosphatidylcholine, phosphatidic acid, phosphatidylinositol, phosphatidylglycerol and choline plasmalogen compared with cells stimulated with adrenaline alone.