The protein and nucleic acid contents in human myometrium from women in the post-menopausal period, from non-pregnant women with ovarian function and from pregnant women at different stages of pregnancy have been determined. The deoxypentose nucleic acid content per mg dry weight is very high in the post-menopausal group, reflecting a considerable nuclear density per unit tissue in this state. Ovarian hormone activity causes a moderate decrease in the density in the non-pregnant group and a considerable reduction during pregnancy. Concomitant and reversely proportional to these changes are the findings of small amounts of protein per tissue unit in the post-menopausal uteri, moderately increasing values in the non-pregnant state and relatively very high values during pregnancy. The pentose nucleic acid content per tissue unit is very low in the post-menopausal group, increases under the influence of ovarian hormones and exhibits very high values during pregnancy. The relative increase during the first 20 weeks of pregnancy is more pronounced than after this period. This finding indicates that the main accumulation of PNA in the cell coincides with the period of rapid growth as judged from the weight-increase of the human uterus. During the normal process of hormone-induced growth, represented by this material, the relative increase in protein and PNA per tissue unit is approximately the same, being about 15 times for the protein and 12 times for the PNA.