Fetal macrosomia in diabetic pregnancy has been shown to accompany increases in the fetal levels of deoxyribonucleic acid (DNA) and proteins in animal model systems. In order to elucidate the underlying mechanisms of this macrosomia, the synthesis of DNA and proteins, the transport of the precursors, and the tissue level of DNA polymerase activities in macrosomic fetuses from mildly diabetic rats were compared with those in normal fetuses on 20.5 and 19.5 days of gestation. Increases in the influx of precursors and stimulation of synthesis of the macromolecules were observed in macrosomic fetuses as early as 19.5 days of gestation. However, stimulation of DNA polymerase activities in macrosomic fetuses did not occur until 20.5 days of gestation. Therefore, stimulation of DNA polymerase may participate in maintenance of macrosomia but does not initiate macrosomia. Whether or not increases in both the influx of substrates and the synthesis activities precede the macrosomia has yet to be determined.