Background: I. Software- and hardwarelimitations of the orthogonal polarisation spectroscopy (OPS) imaging based videomicroscope (VM) CytoScan® A/R made microcirculation measuring in rat liver difficult. To terminate these limitations the video data processing of the VM has been thoroughly analysed. II. Liver resection with acute reduction of sinusoidal cross-section under steady blood flow causes prehepatically an inflow obstruction with acute portal hypertension together with hyperperfused sinusoids in the remnant liver. To determine the regulation of liver blood flow and sinusoidal microcirculation, the sinusoidal hyperperfusion in the same rat liver during stepwise resection has been analysed. Aims: I. Improvement of the measuring method by optimizing the acquisition and analysis of video data. II. Visualization and investigation of the hepatic microcirculation regulation during stepwise liver resection and extensive body circulation monitoring. Material & Methods: I. The actual state of the measuring method has been analysed and a customized state has been defined. The single aims and their adaptive strategies have been determined. II. Stepwise liver resection including caudate resection in the same rat liver (n=6) according to a new, vessel oriented resection technique (Madrahimov, N et al., 2006). The vital parameters have been measured after multiple vessel catheterisation, the microcirculation by means of OPS Imaging prior resection and after each resection step (n=6). The off-line analysis of the sinusoidal diameter SD and the intersinusoidal diameter ISD of midzonal sinusoids as well as the functional capillary density FCD of all perfused vessels has been performed. Results: I. The linkage of the VM with the outsourcing of the video data processing on an external, self-designed PC resulted in a visibly improved and accelerated data acquisition. The microcirculation analysis software of the VM has been replaced by an external and more powerful analysis software. II. Intraoperatively up to 70% partial hepatectomy, it has been detected a linear increasing hypovolemia of the body circulation with a decline of the arterial blood pressure and cava vein flow together with a relatively constant central vein pressure, cava vein pressure, heart and respiration frequency. The reduction of the sinusoidal cross-section led to a portal hypertension and a lowered liver perfusion. The microcirculation analysis revealed an initially low, however clearly increasing sinusoidal dilatation after 90% partial hepatectomy. The ISD decreased continuously with a constant mean FCD and an in each case enlarged standard deviation. Extended resection showed a higher number of inhomogeneously perfused liver lobules. Discussion & Conclusion: It is a known fact, that presinusoidal inlet sphincters are capable of regulating the sinusoidal perfusion. The strong sinusoidal dilatation after 90% partial hepatectomy can be most likely explained by a decompensation of this regulation.