Evidence from studies employing radioactively labelled fatty acids that the stimulation of flux through the diacylglycerol pool is an early action of vasopressin on hepatocytes.

1. In isolated hepatocytes prelabelled with [14C]-arachidonic, -stearic, -linoleic, -oleic or -palmitic acids, vasopressin increased the amount of radioactivity present in diacylglycerols. The largest increase was observed in cells labelled with arachidonic or stearic acids. 2. In cells prelabelled with [14C]- or [3H]-arachidonic acid, the onset of the increase in radioactivity in diacylglycerols induced by vasopressin was slow, the increase was partly dependent on the presence of extracellular Ca2+, and was associated with an increase in radioactivity present in phosphatidic acid which was more rapid in onset. Vasopressin decreased the amount of [3H]arachidonyl-phosphatidylinositol 4,5-bisphosphate, but the magnitude of this decrease was less than 10% of the observed increase in radioactivity in [3H]arachidonyl-diacylglycerol. 3. The concentration of vasopressin which gave half-maximal increase in [14C]arachidonyl-diacylglycerol at low extracellular Ca2+ was 10-fold higher than that which gave half-maximal stimulation of 45Ca2+ efflux. Phenylephrine, but not glucagon, also increased the amount of [14C]arachidonyl-diacylglycerol. 4. It is concluded that an early action of vasopressin on the liver cell is to increase the flux of carbon from phospholipids, including the phosphoinositides, to diacylglycerols.