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.