In a case of reversible paracetamol intoxication ultrastructural changes were correlated to changes in tests of microsomal and cytosolic liver function: the prothrombin index (PP) and the galactose elimination capacity (GEC), respectively. The decrease in the PP was at maximum after 2 days and in the GEC after 5 days, when the PP was normalized. Liver biopsies were taken after 1 and 5 days. Electron microscopy of the early biopsy revealed midzonal increased vacuolization of the endoplasmic reticulum (ER), detachment of ribosomes, and centrilobular mitochondrial injury. The late biopsy exhibited midzonal organelle-depleted areas with pale glycogen rosettes, perinuclear proliferating granulated ER, and centrilobular frank necrosis. These observations are in accordance with the concept that paracetamol-induced hepatocellular injury is due to damage to the membranes of the ER and indicate that the chronological dissociation of the liver function tests reflects the time sequence of the structural changes.