Change in the relative importance of oestrone and oestradiol synthesis by the rat ovary between fetal and prepubertal stages

Ovaries of rat fetuses at 20 and 21 days and of neonatal rats at 5 and 14 days were cultured in the presence of [3H]testosterone, and the conversion percentages into oestrone (E1) and oestradiol (E2) were determined by double isotopic dilution combined with recrystallization to constant specific activity. Insignificant in the 20- or 21-day-old fetus, oestradiol synthesis increased relative to oestrone synthesis in the 5-day-old neonate (E1/E2 = 3.4) and still more at the stage of 14 days (E1/E2 = 0.78). FSH had no effect on oestrogen synthesis at the 4 stages investigated.