The results of several measurements of optical tissue properties often show a wide range of variety. This could be a reason for neglecting time-dependent alterations after the preparation or during the measurement procedure itself. To combine the opportunities optical tomography offers (i.e. structural and functional imaging) with two- or even three-dimensional spectroscopy without using any contrast means of fluorescence dyes, we measured several transmission spectra of various tissues of biological specimen. To prove reproducibility three consecutive measurements were carried out. Generally, a variation in the transmission or extinction could be seen. Their value and direction usually depends on the type of biological specimen. Living cells, freshly taken from the nutritive medium, showed a significant decrease in transmission in contrast to various types of originally frozen animal tissue, which revealed an increase in transmission. But here the value of increase was found to be dependent on the tissue, too. Furthermore, we have made investigations on the dependence of transmission alterations by changing the tissue preparation, e.g. the cutting area in regard to tissue properties. In general, the relative spectral response remained the same, thus resulting in a shift to higher or lower transmissions. The investigations were made with tissue of a thicknesses between 300 micrometers and 1 mm. Details will be explained in this paper. These measurements allow the conclusion, that thin biological specimen are subject to changes of optical properties at the time after having taken them out of a stable in vitro state. These changes are probably due to varying anisotropy. Finally, this leads to the conclusion, that for thin biological specimen the rules of geometrical optics have to be preferred rather than making radiation transport calculations. Optical rules should be part of common methods to determine optical properties of tissue.