Abstract The effect of the tumour promotor 12- O-tetradecanoyl-phorbol-13-acetate on 32P i incorporation into the phosphatidylcholine fraction in mouse epidermis was investigated. 1. 1. Two peaks in incorporation are observed, one at 4–6 h and one at 48 h after treatment. 2. The incorporation [ 3 H] thymidine into DNA of mouse epidermis is inhibited for about 10 h after administration of 12- O-tetradecanoyl-phorbol-13-acetate. At the same time, however, the incorporation of 32P i into epidermal DNA is rapidly stimulated, suggesting that the specific activity of the intracellular ATP pool is increased by the tumour promotor soon after its application. The first peak in 23P i incorporation into phosphotidylcholine may therefore be attributed in part to pool changes. 3. 3. After a short lag phase, the accumulation of phosphatidylcholine in mouse epidermis is stimulated. The ratio phosphatidylcholine/DNA in the epidermis reaches a maximum around 8 h after treatment. 4. 4. Isolated mouse skin pieces pretreated with 12- O-tetradecanoyl-phorbol-13-acetate are capable of incorporating 32P i into the phosphatidylcholine fraction in a manner which is similar to that observed in vivo. The early accumulation of phosphatidylcholine within the epidermis is therefore due to the localised stimulation of the skin microsomal enzymes responsible for its synthesis. 5. 5. The role played by the early 12- O-tetradecanoyl-phorbol-13-acetate-induced stimulation of phospholipid metabolism in the development of tumours during initiation-promotion experiments in mouse skin is discussed.