Androgen receptor function is required for male embryonic sexual differentiation, pubertal development and the regulation of spermatogenesis in mammals. During spermatogenesis, this requirement is thought to be mediated by Sertoli cells and its genetic and pharmacological disruption is manifested in spermatocytes as meiotic arrest. Through studies of a hypomorphic and conditional allele of the androgen receptor (Ar) gene, we have uncovered a dual post-meiotic requirement for androgen receptor activity during male germ cell differentiation. Observations in Ar hypomorphic animals demonstrate that terminal differentiation of spermatids and their release from the seminiferous epithelium is AR dependent and maximally sensitive to AR depletion within the testis. Cell-specific disruption of Ar in Sertoli cells of hypomorphic animals further shows that progression of late-round spermatids to elongating steps is sensitive to loss of Sertoli cell AR function, but that progression through meiosis and early-round spermatid differentiation are surprisingly unaffected.