Bombesin caused a marked stimulation of 32Pi into phosphatidylinositol (Pl), with no apparent lag, and into phosphatidylcholine (PC), after a lag of about 20 min. Stimualtion was blocked by the bombesin receptor antagonist, [D‐Arg1, D‐Pro2, D‐Trp7,9, Leu11] substance P, indicating that the effects on both Pl and PC were mediated through the same receptor. The tumor‐promoting phorbol ester 12‐0‐tetradecanoylphorbol‐13‐acetate (TPA) and dioctanoylglycerol (diC8) both directly activate protein kinase C and in this report were shown to stimulate 32Pi incorporation into PC but not into Pl. In addition, TPA stimulated the release of [3H]choline and [3H]phosphocholine and the accumulation of [3H]diacylglycerol from prelabelled cells. These results strongly suggest that TPA activates a phospholipase C specific for PC. Pretreatment of cells with phorbol‐12, 13‐dibutyrate (PDBu) for 24 h depleted cellular protein kinase C activity and inhibited the ability of TPA to induce these effects suggesting a direct involvement of protein kinase C. Similarly the bombesin stimulation of 32Pi into PC and of [3H]choline and [3H]phosphocholine release was inhibited by PDBu pretreatment. DiC8 and, to a lesser extent, TPA stimulated the translocation of CTP:phosphocholine cytidylyltransferase from the cytosolic to the particulate fraction. DiC8 also stimulated this translocation in cells depleted of protein kinase C. It was concluded that both bombesin and TPA activated protein kinase C leading to activation of a phospholipase C specific for PC.