Spats1 encodes the first reported testis-specific protein containing a long serine stretch. Besides, it bears a probable bipartite nuclear localization signal. Here, we describe the expression pattern of Spats1 in rat along embryonic and postnatal testis development by immunoblots and confocal immunohistochemistry. Spats1 is first expressed in the embryo at 17.5 days post-coitum, coinciding with the time when gonocytes acquire a quiescent state. At this time expression is detected in peritubular myoid cells and gonocytes. Spats1 attains maximum levels during meiosis of the first spermatogenic wave, mainly in pachytene spermatocytes, while a lower signal is also observed in spermatogonia, Sertoli cells and myoid cells. Protein levels dramatically decay afterwards, with minimum expression in adult individuals, where no signal was detected in elongating spermatids or spermatozoa. Spats1 is mostly cytoplasmic, although in pachytene spermatocytes it mapped to nuclei as well. Alkaline phosphatase treatment showed that this protein would be highly phosphorylated. Moreover, we show that the protein is highly conserved along metazoan evolution. Our results suggest a role in the initiation of the first spermatogenic wave, and in the establishment or progression of the first male meiotic division.