Testosterone (T) is essential for spermatogenesis, fertility, and maintenance of the male phenotype. We analyzed in hypogonadal LH receptor knockout (LuRKO) male mice whether T treatment can restore their phenotype, spermatogenesis, and fertility. In LuRKO mice, spermatogenesis is arrested at round spermatids, adult-type Leydig cells are absent, T production is dramatically decreased, the animals are cryptorchid, and their accessory sex organs are atrophic. T replacement therapy from 21 d of life for 60 or 120 d in LuRKO mice induced a male phenotype macroscopically indistinguishable from that of wild-type littermates as well as full spermatogenesis and testicular descent. Thus, the absence of LH-dependent prepubertal androgen priming is not necessary for subsequent maturation of the male phenotype. Conspicuously, some abnormalities remained in epididymal histology after T treatment despite normal expression of several epididymis-specific genes in caput epididymis. The mice displayed normal mating behavior, although at lower frequency than wild-type controls. The spermatozoa were able to fertilize oocytes, but their impaired passage from epididymis to uterus was apparent. The mice remained subfertile, because only 9% of all breedings resulted in pregnancy, and only two of 13 mice (15%) were fertile. Moreover, inflammation in epididymides and prostate was found in many T-treated LuRKO mice, which probably impaired sperm transport and contributed to their high rate of subfertility. In conclusion, T replacement initiated prepubertally only partially restores the fertility of LuRKO mice, even though most features of the male phenotype recover. Full fertility may require higher and/or earlier postnatal T exposure or production of other Leydig cell factors lacking in this model.