3H-progesterone uptakes by the ovaries and skeletal muscles were compared in newborn female Wistar rats and in other females aged five, ten, 15, 20, 25, and 50 days, two hours after one single intraperitoneal injection of 50 muCi/100 g body weight. The ratio of the concentration of radioactivity between the ovaries and the skeletal muscle increased significantly at the age of 15 days. The increase in ovary/skeletal muscle raio (characterised by a logistic curve) suggests the first appearance of specific binding capacity for progesterone, coinciding with an increase in plasma progesterone level and beginning of the distince atretic changes in compact and cavitated follicles simultaneously with the attainment of capability to ovulate following adequate exogenous gonadotropic stimulation. The early postnatal period of protective short-term treatment, using progesterone, against the early androgen or oestrogen syndrome of anovulatory sterility and/or noxious long-term treatment, using progesterone, inducing a delayed anovulatory syndrome precedes the beginning of preferential accumulation of 3H-progesterone in the ovary.