The critical period of development is highly susceptible to disorders. Environmental contaminants, stress, and poor nutrition may permanently affect structurally and functionally an organism during adulthood. Protein restriction in intrauterine and neonatal periods may impair testicular cells and reduce steroidogenic activity. The current study investigated the effect of low protein diet during intrauterine and postnatal life on testicular function in immature and adult rats. Pregnant Wistar rats were fed either a normal protein diet or a low protein diet during pregnancy until birth and during lactation until weaning. The results showed that undernutrition during intrauterine life reduced epithelium height, tubular volume, Leydig cell volume, and serum testosterone levels, although it increased the support capacity of Sertoli cells. Tubular diameter, tubular volume, epithelium height, and Leydig cell population decreased when protein was restricted during lactation. Pregnant rats submitted to a low protein diet during the two periods generated male rats with a reduction in seminiferous tubule volume and length, Sertoli cell population, Leydig cell population and volume, and sperm production. Undernutrition during both the intrauterine and postnatal periods caused the most drastic testicular effects, as irreversible damage to Sertoli cell population and, consequently, to sperm production in the adult male.