Sixteen female, Sprague-Dawley rats were divided into four equal groups. Two groups served as controls receiving low or regular concentrations of fluoride (F); animals in the other two groups were given drinking water, containing 100 parts/10(6) F, for 3 weeks either during or immediately before pregnancy. Thirteen days after delivery, the pups and dams were killed and various tissues analysed for F content. Prenatal F supplementation increased F concentrations in plasma, mandibular incisors and femoral epiphyses of pups by 25, 36 and 38% respectively, when given during pregnancy. Only a slight increase of 9 and 11% in the respective F concentrations of incisors and epiphyses occurred when the supplement was given before pregnancy. The fluoride level of milk was consistently higher than that of the maternal plasma. These results suggest the need for further study of prenatal F supplementation.