There is accumulating evidence that the mammalian Y chromosome, in addition to its testis-determining function, may have other male limited functions, particularly in spermatogenesis. We have previously shown that the short arm of the mouse Y carries information needed for spermatogonial proliferation. This information, together with the testis-determining gene Sry, is contained within the Y-derived sex reversal factor Sxra. XO males carrying a copy of Sxra attached to the X chromosome are nevertheless sterile owing to an almost complete arrest during the meiotic metaphase stages. Here we show that this meiotic block can be overcome by providing a meiotic pairing partner (with no Y-specific DNA) for the XSxra chromosome. However, this does not restore fertility because the sperm produced all have abnormal heads. It is concluded that the Y-specific region of the mouse Y chromosome long arm includes information essential for the normal development of the sperm head.