The synaptonemal complex proteins SCP3 and SCP1 are components of the synaptonemal complex, a meiosis-specific protein structure essential for synapsis of homologous chromosomes. Using polyclonal antibodies raised against SCPs of rat testis, we have studied the expression of these proteins in embryonic germ cells of the mouse embryo using immunohistochemistry and immunoblotting. This investigation provided the first description of the sequential appearance of SCP3 and SCP1 during the different stages of the female meiosis in the mouse. Most importantly, we found that also male primordial germ cells express SCP3 for a short time before undergoing G1 arrest. This strongly supports the hypothesis that primordial germ cells are programmed to enter meiosis unrespective of the sex and that foetal testis produces a factor that inhibits such programme.