The measurement of the partial pressure of oxygen in the tissue is a method increasingly used in experiment and clinic, the technical prerequisites for this having been created by means of the development of polarographic measuring techniques (surface or scarifier-electrodes). The pO2-measurement of the tissue renders possible independent conclusions to the relations of microcirculation of the organs examined and to the effectiveness of the blood supply of the tissue as well as its homogeneity and nutritive achievement for the cellular metabolism. On the basis of the intravital microscopic findings in connection with perivascular pO2-measurement the pathophysiological peculiarities of this measuring method are discussed. The physiologically involved temporary and spatial inhomogeneities of the peripheral blood supply limit, however, the generalization of locally (approximately 200-300 microns3 measuring area) established oxygen values for diagnostics and therapy control. For the measurement an intact microcirculation and the simultaneous registration in several measuring places is necessary. Then the following informations may be obtained, e.g.: control of the exact dosage of an inspiratory O2-supply--control of the volume balance--control of the haemodynamic effect of drugs on the microcirculation.