In eukaryotes, many receptor agonists use phospholipase-generated lipids as intracellular messengers. Receptor occupation stimulates the production of polyunsaturated 1,2-diacylglycerols by phosphatidylinositol-4,5-bisphosphate specific phospholipases C and/or of mono-unsaturated and saturated phosphatidates by phospholipase-D-catalysed phosphatidylcholine breakdown. The primary phospholipase products are rapidly metabolized: polyunsaturated 1,2-diacylglycerols are converted to polyunsaturated phosphatidates by diacylglycerol kinase; mono-unsaturated and saturated phosphatidates are dephosphorylated to give mono-unsaturated and saturated 1,2-diacylglycerols by phosphatidate phosphohydrolase. The phospholipase-generated polyunsaturated 1,2-diacylglycerols and mono-unsaturated and saturated phosphatidates appear to be intracellular messengers, whereas their immediate metabolites probably do not have signalling functions.