The fixation of rat liver by perfusion with glutaraldehyde with different pressures has been investigated. For this study adult male albino rats were used. Rat livers were fixed by perfusion through the abdominal aorta according to the method of FORSSMANN et al. (1967). Perfusion pressures varied from 30 to 210 mmHg. A continuous complete endothelial lining of liver sinusoids could be visualized with TEM and SEM after fixation with perfusion pressures lower than 100 mmHg. Three different regions could be noticed in the endothelial cell: 1. prominent nucleous region, 2. compact cytoplasmic processions containing mitochondria and ergastoplasma, 3. delicate fenestrated cytoplasmic areals. As a rule the fenestrations were localized in groups, s.c. sieve plates. After perfusion fixation with pressures above 100 mmHg the endothelial lining of liver sinusoids appeared similar to a wide-meshed net. The sieve plates were destroyed, and numerous defects could be found in the endothelial cells. Hepatocytes showed vacuoles which seem to be due to invagination of the cellular membrane. For the development of artifacts even with physiological perfusion pressures in the aorta (110 mmHg), the content of procaine in the rinsing solution is responsible. Eliminating the function of arteriols leads to unphysiological pressure effects in the sinusoids.