The incorporation of 14C-labeled L-aspartic acid, L-glutamine, glycine, L-glutamic acid and D-glucose into the fetal and placental RNAs was studied using 14-day pregnant rats. Although all five compounds demonstrated some degree of incorporation, D-14C-glucose was the most effective precursor for both the fetal and placental RNAs. Transfer studies of D-glucose demonstrated a facile transplacental passage, and radiochromatogram analyses showed that D-14C-glucose was incorporated into both the ribose moiety and the nucleic acid bases. In contrast to our previous studies with the near term pregnant rats, there was a normal 18 S radioactivity peak in the placenta RNA resulting from changes in the technique used to extract the placental RNA.