Phenobarbital administration to pregnant rats from day 12 to day 19 of gestation suppressed body weight gain and produced significant effects on reproductive function in their offspring. These effects included delays in the onset of puberty, disorders in the estrous cycle, and infertility. Moreover, the animals exposed to phenobarbital in utero showed altered concentrations of sex steroids, gonadotrophic hormones, and estrogen receptors. These findings suggest that phenobarbital treatment during prenatal development can produce permanent alterations in sexual maturation.