Several mechanisms of agonist control of PC hydrolysis have been described: control by G proteins, protein kinase C, Ca2+, and tyrosine kinases. The relative importance of these mechanisms remains to be demonstrated. Another major point is the physiological significance of PC hydrolysis. The prolonged formation of DAG from PC may be important in cellular control mechanisms that require long term activation of protein kinase C. The functions of PA are presently unknown, but the rapid formation of high concentrations of this lipid during stimulation by agonists strongly suggests that it has signaling functions. It is obvious that much work remains to define the physiological significance of agonist-stimulated PC breakdown.