Autoradiograms of histological specimens from the cervix of seventy-five mice were analysed after in vivo injection with an RNA precursor. All nucleated, non-pyknotic cells and some mitotic figures were labelled, which suggested that the tissue was metabolically active (i.e. viable). Only the uppermost epithelial pyknotic cells were devitalized, as deduced by the morphological appearance of the cells and by the absence of label. The viability of the whole epithelium suggests that the previously reported focal distribution of proliferating and non-proliferating areas in the cervical epithelium of mice is a genuine phenomenon.