This study investigated the pre- and postnatal effects of protein restriction (8% vs 20% crude protein) on different parameters of spermatogenesis in adult rat offspring. Body and testis weights as well as the seminiferous tubular diameter were reduced in those animals that received the protein-restricted diet after weaning, although these parameters recovered when a 20% protein diet was offered subsequently. The numbers of spermatogonia, spermatocytes, spermatids and Leydig cells were reduced in undernourished animals, whilst the Sertoli cell number did not change. Prenatal programming effect was observed only in the spermatogonial or proliferative phase of spermatogenesis. However, the intake of the normal protein diet after weaning brought many of the testicular parameters evaluated back to normal in 70-day-old rats. A significant reduction of the meiotic index, Sertoli cell supporting capacity and spermatogenic efficiency was observed in animals subjected to protein undernutrition throughout their lives. The data presented show that protein restriction impairs the normal development of the testis in different ways, depending on the period during which the restriction was imposed, and the negative effects on spermatogenesis are more severe when undernutrition occurs from conception to adulthood; however, the return to a normal protein diet after weaning recovers the spermatogenic process.