Rats do not respond to human chorionic gonadotrophin (HCG) in the first days of life: oestrogen is apparently not secreted after treatment with HCG until the animals are 8–9 days old (Freed & Soskin, 1937; Falck, 1953). Similar results have been obtained in our colony of Wistar rats (Divila, Presl, Krabec & Horský, 1971). An attempt was made to study the relationship of the age-dependent stage of rat ovarian development and the refractory condition of the ovary with the beginning of its responsiveness to HCG, by means of an autoradiographic technique after injection of 125I-labelled HCG. Human chorionic gonadotrophin (Organon, Oss; biological activity of 8000 i.u./mg) was iodinated by the method of Greenwood, Hunter & Glover (1963) using 125I (The Radiochemical Centre, Amersham). The average specific activity was 35 μCi/μg protein. Groups, each of five Wistar rats aged 5–30 days, were injected i.p. with a single dose of
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Presl et al. (1972) studied this question.