In the course of a study of circadian rhythmicity in the number of cells in the S-phase of the cell cycle in mouse kidney, a comparison was made between 2 methods of estimation of DNA synthetic activity. One of the methods used was an autoradiographical analysis of the tritiated thymidine labelling index; the other was a biochemical assay of the DNA content of the tissues coupled with liquid scintillation counting, to obtain an estimate of the incorporation of tritiated thymidine, expressed as dpm per microgram of DNA. The correlation between individual estimates of DNA-synthetic activity, using the 2 methods was highly significant, as was the correlation between the 2 data sets with respect to time of zeniths. There was a suggestion that the biochemical assay procedure may be the more sensitive indicator of circadian rhythmicity in relatively homogeneous tissues with low proliferative rates. Estimated over a 3-da period, the time of maximal DNA synthetic activity lay between the hours of 22.00 and 02.00. The nadir of the curve was less well defined and occurred between 02.00 and 14.00 hours. The autoradiographical study took 5 weeks to complete exclusive of histological and autoradiographic preparation. The biochemical assay was completed in 4 days.

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