The occurrence and distribution pattern of spontaneous single-strand breaks (nicks) and/or gaps of mouse chromosomal DNA were studied with the help of nick-translation procedure omitting exogenous nucleases. The holoenzyme and a Klenow's fragment were used at a concentration of 0.I. U/20 microl of reaction mixture, resp. Bio-dUTP and streptavidin-alkaline phosphatase were used for labeling and detection. Chromosomes of postimplantation embryos and bone marrow were not stained. Chromosomes of all preimplantation stages of development were homogeneously stained with prominent dots of various size and intensities of grayish. DNA Pol I and the Klenow enzyme demonstrated a similar pattern of labelling. The centromeric heterochromatin was not labeled. The label was localized asymmetrically exclusive of NOR and telomeric regions.