In order to estimate the sensitivity of macroautophagy in liver toward changes in ATP we have analyzed the volume density of the autophagic/lysosomal system in isolated rat hepatocytes, incubated under conditions where intracellular ATP was partially depleted. (a) It appeared that reduction of the intracellular ATP concentration by 30-50% decreased the volume density of autophagic vacuoles by 70%. (b) Partial ATP depletion did not involve significant changes in the volume density of dense bodies. Together with studies showing that the rate of overall proteolysis via macroautophagy decreases with decreasing ATP concentration (P.J.A.M. Plomp, E.J. Wolvetang, A.K. Groen, A.J. Meijer, P.B. Gordon, and P.O. Seglen (1987) Eur. J. Biochem. 164, 197-203) our data indicate that changes in intracellular ATP primarily affect early steps in the autophagic/proteolytic pathway.