Carefully performed pulse titration of the isolated rat live in the course of continuous erythrocyte free perfusion with small amounts of digitonin causes a short term permeability of liver cell plasma membranes with concomitant short lived release of intracellular low or high molecular compounds such as ATP or lactate dehydrogenase. Gluconeogenesis from lactate being completely inhibited during this period restores within about one minute up to a level that depends on the amount of perfused digitonin. The described experimental model is suggested to be useful for the measurement of cytoplasmic metabolites under physiological conditions. It moreover offers the possibility to import foreign substances into liver cells that normally do not penetrate liver cell plasma membranes.