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To determine changes in steroidogenesis by rat ovaries during sexual maturation, ovaries obtained at various ages (days 10–35) and at the first pro-oestrus were incubated in the absence or presence of LH and the accumulation of steroids in the medium was measured.

Basal and LH-stimulated oestradiol-17β and testosterone release into the medium, expressed in pmol/4 h per mg ovary, was high at day 10 of age and at first pro-oestrus. Between days 20 and 35 basal oestradiol and testosterone release was low and could not be stimulated by LH. Addition of testosterone to the culture medium increased oestradiol production at all ages studied. Release of progesterone occurred at all ages even in LH-free medium. Incubation in the presence of LH resulted in a dose-dependent increase in progesterone with a maximal response at pro-oestrus. Androsterone and 5-androstane-3,17β-diol production in the absence or presence of LH was high during the entire prepuberal period. Production of 5-reduced androgens in response to LH increased from days 10 to 20 but decreased thereafter. Similarly, 5-reductase activity, measured in ovarian homogenates, increased from days 10 to 20 but was decreased again by first pro-oestrus. A further decrease in basal and LH-stimulated 5-reduced androgen production occurred after first ovulation.

These results demonstrated age-related changes in steroid release after in-vitro incubation. At day 10 progesterone can be converted to aromatizable androgens allowing production of oestrogens, while after day 10 progesterone is converted to 5-reduced C₁₉ steroids. The decrease in 5-reductase activity correlates with an increase in LH-stimulated testosterone and oestradiol production at the first pro-oestrus.

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