The important influence of postnatal thyroid status on development is well recognized, while the effect of prenatal thyroid status on postnatal growth is less understood. The present study was undertaken to assess the effects of pre- and postnatal thiouracil administration on postnatal growth and pituitary grown hormone (GH) content in 25 day old rats. Pups were born to Sprague-Dawley rats fed lab chow mash containing 0.25% thiouracil according to the following schedules: a) during gestation only; b) during lactation only (25 days postpartum); and c) through gestation and lactation. Mothers of euthyroid controls were fed diet without thiouracil for the length of the experiment. Pups were weighed daily through day 21 and decapitated on day 25. Pituitary GH content was measured by densiometric comparison of disc gel column electrophoresed pituitary homogenates with similarly treated rat GH standard. Incorporation of thiouracil into the diet of pregnant and/or lactating rats significantly depressed the body weight and pituitary GH levels in their 25 day old pups. Furthermore, analysis of the daily mean body weight of each litter revealed four significantly different patterns of growth. While previous investigators have reported that prenatal thyroid restriction does not significantly alter the developmental processes of young rodents, the present study indicates that prenatal thyroid status is an important influence on normal postnatal growth.