Phosphatidylinositol-4-monophosphate and phosphatidylinositol-4,5-bisphosphate increased the activity of the vanadate-sensitive ATPase associated with plasma membranes isolated from both sunflower hypocotyls and carrot suspension culture cells. The response was not due to the metabolism of the polyphosphoinositides since diacylglycerol, inositol-1,4-bisphosphate, inositol-1,4,5-trisphosphate, glycerophosphoinositol monophosphate and glycerophosphoinositol bisphosphate had no effect. These data suggest that activation of the inositol phospholipid kinases could be a critical step in signal transduction in plants.