Interferometric and morphometric studies were performed on seven types of brain cells of deparaffinised and Feulgen-stained brain sections of pre- and postnatal mice in order to find out to what extent 3H-beta-self-absorption factors (c.f.s) of karyoplasm and perikaryal cytoplasm change as the animals' age increases. This is important when comparing grain numbers of different cell types among mice of different age in quantitative autoradiographic studies after application of tritium-labelled substances. While optical path differences (o.p.d.s) of euchromatin and cytoplasm decreased in large neurons postnatally, heterochromatic o.p.d.s remained more or less unchanged. The fraction of euchromatin (fEu) and heterochromatin (fHe) changed specifically in relation to cell type with increasing age, i.e. mostly heterochromatin increased postnatally. Within a given age, c.f.s, calculated from o.p.d.s, fEu and fHe, varied up to a maximum factor of 2.16 for karyoplasm and a factor of 1.19 for cytoplasm between different cell types. A significant decrease of c.f.s postnatally was found only for the cytoplasm of cortical and hippocampal pyramidal cells and for cerebellar Purkinje cells. The c.f.s of karyoplasm remained more or less unchanged with increasing age of the mice. The calculation of relative c.f.s makes these c.f.s applicable for every other autoradiographic study using 3H-labelled substances.