Human endometrial and myometrial tissues were incubated in vitro with [3H]oestradiol-17\g=b\ and [3H]oestrone to study the uptake and interconversion of these two steroids by the tissues. Endometrial tissue displayed a higher capacity for oestrogen interconversion than myometrial tissue and the oxidation of oestradiol-17\g=b\ to oestrone was favoured, rather than the reverse reaction. A greater degree of oxidation was found in tissue taken from uteri in the secretory phase than in the proliferative phase. A study of the distribution of radiometabolites between 'soluble' and 'particulate' fractions of tissue homogenates showed that a greater proportion of the tissue radioactivity was associated with the 'particulate' fraction in the proliferative phase than in the secretory phase. After incubation of endometrial tissue from the secretory phase with tritiated oestrogens there was evidence of the formation of chloroform-insoluble radiometabolites and some of these were tentatively identified as oestrogen sulphates. In a second experiment, the uptake and metabolism of the same two oestrogens by tissue from leiomyomata uteri (fibroids) and normal myometrial tissue were compared. No significant difference between these tissues was found.The results suggest that the levels of 17\g=b\-hydroxysteroiddehydrogenase in the human uterus may be dependent upon the levels of oestrogens in the blood. The lack of a difference in the treatment of oestrogens by fibroid and normal myometrial tissue suggests that these tissues may have a similar mechanism for the uptake and retention of oestrogens.

To determine whether the established capability of rabbit uterine tissue to interconvert 17 p-oestradiol and oestrone might have some effect on the mode of action of the oestrogens in this organ, the in vitro interconversion of [3H]-17p-oestradiol and [3H]oestrone by rabbit endometrial and myometrial tissue was investigated and the identity of radiometabolites in 'soluble, 'mitochondrial-microsomal' a'nd 'nuclear' preparations was studied. Both endometrial and myometrial tissue were found to be capable of oxidoreduction of the oestrogens, the equilibrium of the reaction favouring the reduction of oestrone. Irrespective of the tissue--steroid combination studied, the greater part of the radioactivity in all fractions was associated with 17p-oestradiol. The relative proportions of oestrone varied between fractions, the nuclear preparation consistently showing a lower proportion of oestrone than the other fractions. Sephade

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