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Functional capillary density (FCD) is one of the parameters obtained by intravital microscopy using epi-illumination of the tissue surface or trans-illumination of thin tissue layers. FCD, defined as the length of red cell-perfused capillaries per observation area (cm-1), has been used as an indicator of the quality of tissue perfusion in various animal models. Quantitative analysis of FCD in randomly selected regions of the tissue is performed by means of a computer-assisted video analysis system which allows calculation of the length of RBC-perfused capillaries. Basically, two different mathematical approaches can be employed: the first approach is based on the addition of the distances between two neighboring points (pixels) on the video screen (Pythagorean principle). The second approach uses the superimposition of a grid system that allows estimation of the capillary length by counting the number of intersections between the capillaries and the grid lines (stereological approach). The immanent error has been calculated in our laboratory to be +/- 1% with the Pythagorean and +/- 5% with the stereological method. Beside these systematic errors of computerized measurement, the individual (user-dependent) errors occurring during recognition and redrawing of the capillaries on the video image with use of a digitizing tablet are in the range of +/- 10% (intraindividual) and +/- 70% (interindividual) for the recognition and +/- 3% (interindividual) for the redrawing procedure. Our studies indicate that the errors resulting from the use of a computer-assisted calculation (Pythagorean or stereological approach) or the user-assisted redrawing of the capillaries are negligible when compared to the errors made during recognition of the capillaries on the video screen. The methods are applied for assessment of FCD in two different microcirculation models of skin muscle and pancreas yielding highly reproducible, user-independent results under physiologic conditions and the pathophysiologic conditions of ischemia-reperfusion.