Mitotic indices in the basal layer of cheek pouch epithelium of Chinese hamsters show a pronounced circadian rhythm. Radiation from tracer doses of tritiated thymidine of high specific activity caused mitotic delay, or temporary interruption of the flow of cells into mitosis. The wave of labeled mitoses method yielded a transit time across S of 10 hours, while double-labeling experiments indicated a synthesis time of about 5 hours. Flow of cells from synthesis through mitosis, upon which the wave of labeled mitoses method depends, was interrupted by mitotic delay from tritiated thymidine, the circadian rhythm, or both, resulting in an overestimate of the duration of S. Data from the double-labeling experiments were analyzed using a steady-state model which depends on total asynchrony in the dividing population of cells; it is not strictly applicable in the presence of a circadian rhythm. These results stress the need for simultaneous multiparameter analysis and the use of several experimental approaches in cytokinetic studies of cell renewal systems in vivo.

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