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Changes in serum and pituitary LH and FSH concentrations have been measured in the newborn male rat before, during, and up to 24 h after birth. A sudden and transient increase of serum and pituitary gonadotropins is observed at birth, which is followed by a rapid increase of absolute and relative testicular weights between 2--12 h (P less than 0.0001) and by a transient increase of serum testosterone between 0 h in utero (810 +/- 26 pg/ml) and 2 h (2820 +/- 318 pg/ml; P less than 0.0001). Similarly, premature newborn rats obtained by cesarian delivery on day 20 of gestation also exhibited an increase in testicular weight between 0--6 h and an increase in serum testosterone levels between 0 h (730 +/- 170 pg/ml) and 2 h (3400 +/- 300 pg/ml; P less than 0.001) with only a slight increase in serum LH. These results show that the hypophyseo-testicular axis of the rats is stimulated at the moment of birth. The factors responsible for this stimulation are discussed. This transient testicular crisis occurring at birth could affect the process of masculinization of the central nervous system of the rat.

These studies were designed to investigate the regulation of testicular testosterone and the role of the hypophysis during the last 5 days of gestation in the rat. Plasma LH-like activity was measured by bioassay and testosterone by radioimmunoassay. In both sexes, plasma LH-like activity levels were low on days 17.5 and 18.5 (< 0. 10\ p=n-\ 0. 26 ng/ml) and rose on day 19.5 (0.10\p=n-\1.25 ng/ml). Thereafter the levels increased considerably and were significantly higher in females than in males: on day 21.5 they reached 0.44\p=n-\2.42 ng/ml in males and 2.20\p=n-\3.96 ng/ml in females. Plasma levels of testosterone were significantly higher in males than in females and were maximal on day 18.5 in males. Testicular content of testosterone reached maximum values on day 18.5 and remained constant thereafter. Foetal decapitation clearly reduced testicular production of testosterone and plasma LH-like activity only after day 19.5. A residual gonadotrophic activity was detected at all stages in decapitated foetuses from both sexes (< 0.10\p=n-\0.46 ng/ml). These results suggest that pituitary LH secretion and pituitary-testicular interrelationships are essentially established between day 19.5 and 20.5, and that the ageing testis becomes more dependent on gonadotrophic stimulation. The existence of an extrapituitary LH-like activity might indicate the presence of placental gonadotrophins.We recently reported that testicular production of testosterone can be decreased by hypophysectomy at the end of gestation in rat foetuses (Naessany et al. 1981). This observation suggests that the foetal hypophysis plays a main role in the control of steroidogenic activity of the testes although clear cut observations were not obtained by other au¬

Testosterone secretion by foetal rat testes (13 1/2-21 1/2 days of gestation) explanted for 3 days in a synthetic medium was measured every 24 h by radioimmunoassay. During the first day of explantation, the foetal testis produced, respectively, 1013 +/- 132, 8734 +/- 1118, 9179 +/- 2185 and 3886 +/- 309 (S.E.M.) pg/testis when explanted at 14 1/2, 16 1/2, 18 1/2 and 21 1/2 days respectively. Testosterone production by 13 1/2-day-old testes was not detectable on the first day of culture, but appeared on subsequent days. Daily testosterone secretion increased on the 2nd and 3rd days of culture in 14 1/2-day-old testes and decreased in older stages. These results suggest that the functional differentiation of the testis is independent of stimulatory factors like gonadotrophins. Dibutyryl cyclic AMP was found to stimulate testosterone production significantly from 14 1/2 days of gestation onwards.

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There is a great annual variation in plasma testosterone in M. murinus. The averages for a dozen males can reach over 52.4 ng/ml in April and 1.97 ng/ml in October. During the breeding season, the values can reach more than 100 ng/ml and are among the highest found in mammals. The reasons for the important individual variations are open to discussion.

A sensitive bioassay for plasma LH-like activity has been developed by utilization of the testosterone response of foetal rat testes to gonadotrophic stimulation in vitro. The volume of plasma required for the assay and the target organ make the method suitable for bioassays of circulating LH-like activity in foetal rat plasma. When applied to foetal plasmas, the method has shown that castration performed on day 19 of gestation induced 24 h later an increase in the LH-like activity of the plasma and that decapitation under the same conditions led to a decrease in this activity in male foetuses.