The role of nutritional factors in the development of prenatal and postnatal growth retardation is not well understood. We tested if thiamine deficiency may cause intrauterine growth retardation in rats. From the second day of gestation Sprague-Dawley rats were freely fed either a nutritionally complete or a thiamine-deficient diet. A similar group of rats was pair-fed with a complete or a thiamine-deficient diet and daily pyrithiamine injections (50 micrograms/100 gm of body weight) were given to precipitate thiamine deficiency during the short gestation of the rat. Maternal thiamine levels in blood and brain tissues, maternal erythrocyte transketolase activity with thiamine pyrophosphate effects, and fetal tissue thiamine levels were measured. The results indicate that feeding a thiamine-deficient diet in conjunction with pyrithiamine injections caused sufficient thiamine deficiency to induce intrauterine growth retardation in the progeny. We conclude that thiamine deficiency alone during in utero development in the rat may contribute to intrauterine growth retardation.