Changes in ovarian oestrogens and in plasma gonadotrophins in female rabbits from birth to adulthood.

Plasma oestrogens (E1, E2) and gonadotrophins, and ovarian oestrogens, were determined in female rabbits from birth to 6 months. Increases in ovarian weight followed a curvilinear pattern, with a phase of slow growth (from 1 to 60 days) preceding a phase of rapid growth (from 60 to 90 days). At birth, E2 was already quantifiable in ovaries; it remained at very low levels up to 50 days. Ovarian E2 content increased until 6 months with two sharp rises between 50 days (46 +/- 4 pg/2 ovaries) and 60 days (365 +/- 53 pg/2 ovaries) and between 80 days (326 +/- 77 pg/2 ovaries) and 90 days (1118 +/- 307 pg/2 ovaries). E1 appeared in ovaries at 50 days and then followed a pattern similar to that of E2. Oestrogens were never detected in plasma. At birth FSH and LH levels were similar to those measured in adult females but from 10 days the secretion of both gonadotrophins showed a clear dissociation. LH concentrations varied little during the period studied (between 1077 +/- 106 pg/ml to 2030 +/- 466 pg/ml). Plasma FSH levels were considerably higher between 10 and 50 days than those measured thereafter, particularly in adulthood. The decline in FSH levels occurred simultaneously with the rise in ovarian oestrogens.