It is currently admitted that Follicle-Stimulating Hormone (FSH) is physiologically involved in the development and function of fetal/neonatal Sertoli cells in the rat but not the mouse. However, FSH is produced by both species from late fetal life onwards. We thus reinvestigated the role of FSH in mouse testis development at day 0 (birth) 6, 8 and 10 post-partum (dpp) by using mice that lack functional FSH receptors (FSH-R(-/-)). At birth, the number and proliferative index of Sertoli cells were significantly lower in FSH-R(-/-) mice than in wild type neonates. Claudin 11 mRNA expression also was significantly reduced in FSH-R(-/-) testes at 0 and 8 dpp, whereas the mRNA levels of other Sertoli cell markers (Transferrin and Desert hedgehog) were comparable in FSH-R(-/-) and wild type testes. Conversely, AMH mRNA and protein levels were higher at birth, comparable at 6 dpp and then significantly lower in FSH-R(-/-) testes at 8-10 dpp in FSH-R(-/-) mice than in controls. Although the plasma concentration of LH and the number of Leydig cells were similar in FSH-R(-/-) and control (wild type), testosterone concentration and P450c17 mRNA expression were significantly increased in FSH-R(-/-) testes at birth. Conversely, at 10 dpp when adult Leydig cells appear, expression of the steroidogenic genes P450scc, P450c17 and StAR was lower in FSH-R(-/-) testes than in controls. In conclusion, our results show that 1) like in the rat, signaling via FSH-R controls Sertoli cell development and function during late fetal life in the mouse as well; 2) paracrine factors produced by Sertoli cells are involved in the FSH-R-dependent regulation of the functions of fetal Leydig cells in late fetal life; and 3) the role of FSH-R signaling changes during the prepubertal period.