Allopregnanolone is the best characterized among neurosteroids, and its role in the control of neuroendocrine axes has attracted increasing interest recently. However, there is no available information about circulating levels of allopregnanolone during infancy, childhood and puberty. We studied two groups of children:1) those aged between 0 and 2 y (n= 72), and2) those aged between 6 and 18 y, at different Tanner's stages (n= 82). In each of these patients, serum allopregnanolone, progesterone, cortisol, and dehydroepiandrosterone levels were evaluated after informed consent; allopregnanolone was measured by RIA after acid extraction on cartridge. There was no significant variation of serum allopregnanolone levels either in male and female children during the first 2 y of life. Furthermore, although serum dehydroepiandrosterone levels showed a significant decrease, inversely correlated with age of the children (p< 0.01), serum cortisol and progesterone levels showed a significant age-related increase during the first 2 y of life. Cortisol and allopregnanolone levels were positively correlated (p< 0.01). During puberty, we observed a progressive increase in serum allopregnanolone levels in both boys and in girls, which were higher at Tanner's stage IV-V (0.7 ± 0.01 nM; mean ± SEM) than at stages I-II (0.32 ± 0.02 nM;p< 0.01); mean levels were significantly higher at puberty than in the first 2 y of life (p< 0.01). Furthermore, during puberty, serum progesterone and dehydroepiandrosterone levels also increased progressively with age in both boys and girls. Allopregnanolone and dehydroepiandrosterone levels were positively correlated throughout puberty. The present results indicate that serum allopregnanolone levels do not change during the first 2 y of life but increase during pubertal development, suggesting that this steroid may be involved in the adaptive neuroendocrine mechanisms related to puberty.