Morphological and sexual differentiation extending far into postnatal life in the rat, a species with short gestation, is influenced in the female first by the absence and later by the presence of ovarian oestrogens. Therefore, in the present study the total oestrogen concentration was determined in the blood of normal female rats from the early postnatal critical period of hypothalamic sexual differentiation until sexual maturity.
Wistar rats aged 5–7 days were used. The animals were given heparin s.c. (5- and 10-day-old animals receiving 500 i.u., all others 1000 i.u.) and were killed by decapitation 45 min. later. At each age studied, blood from groups of 6–120 animals (Table 1) was pooled and replicate samples of 5 ml. were taken from each pool. The oestrogens were extracted and purified according to the method described in detail by Herzmann, Presl, Horský & Vrchlabská (1967) and were measured fluorimetrically (Ittrich, 1960).