Autoradiographs of adult mice killed at various times after injection of tritiated thymidine show significant numbers of labelled nuclei in organs in which mitoses are either very rare or completely absent. The proportion of labelled cells that divide was estimated from the decline in the number of grains per nucleus, the number of pairs of labelled cells in sheets of epithelium in squashes, the number of labelled metaphases after 6 hours' treatment with Colcemid, and the ratio of mitotic index to labelling index. The longest possible duration of G2 in the epithelial cells of seminal vesicles was deduced from the results of Feulgen photometry. The results show that only a small proportion of the labelled cells divide in the seminal vesicles and liver, whilst probably none divide in brain, smooth muscle, and heart muscle. It is suggested that, in the so called "non-dividing tissues" of adult mice, cells periodically renew their DNA by a process the details of which are as yet unknown.

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