The aim of the study was to assess the effect of age of the animal upon the real thickness of the frozen sections. The study was performed on 19 rabbit brains. The thickness of the frozen sections regardless of their staining is age-dependent. The relation is proportional during the period from 7 to 180 postnatal day and characterizes both immunohistochemical as well as cresyl violet-stained sections; moreover, changes of the section thickness proceed parallelly. It is suggested that especially for some stereological parameters all required procedures should be standardized to achieve comparable and unbiasedly interpretable results.