In a total of 120 rats the number of3H-Tdr-labelled nuclei of the adrenocortex was counted during various phases of the estrous cycle. In two separate experiments cyclic variations in the number of DNA synthesizing cells were found. The labelling index of the zona glomerulosa showed a significant maximum in proestrus whereas in zona fasciculata a minimum was revealed in estrus. In the glomerulosa the maximum of labelled cells was followed by a mitotic peak in estrus. Due to very low numbers of labelled cells no rhythm was found in the reticularis. The possible endocrine regulations of this endogenous rhythm of adrenocortical cell replication are discussed.
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