Following injection of U-14C threonine into the haemolymph of adult female Glossina morsitans during late pregnancy, radioactivity was detected in the postparturient female and in its offspring, in threonine, lipids, and a range of non-essential amino acids. The level of radioactivity recovered from the larva was higher than that remaining in the injected adult, and the radioactivity recovered was considerably higher in the amino acid than in the lipid fraction. Administration of labelled threonine into maternal haemolymph on each of the first 8 days of the 9-10 day long pregnancy cycle was followed 24 h later by measurement of radioactivity in the developing oöcyte and in the intra-uterine progeny. The patterns of nutrient uptake are discussed in relation to vitellogenesis in the oöcyte and to growth of the larva. Analysis of the expired carbon dioxide and excreta was carried out 24 h after maternal injection of labelled threonine on the first or eighth day of pregnancy. Carbon dioxide and excreta from females in early pregnancy showed significantly higher radioactivity than those from females in late pregnancy. In both cases, radioactivity in the amino acid fraction from the excreta was extremely small and about 95% of the total activity was in uric acid. These results are discussed in terms of the utilization of threonine in relation to the metabolic demands for various nutriments by the pregnant female.