Plasma lipidic components appear augmented in 19- and 21-day pregnant rats compared with virgin controls, the greatest effect being in triglycerides, corresponding to the augmented proportion of plasma very low density lipoproteins (VLDL). The plasma lipidic components in fetuses of 19 and 21 days are lower than in their mothers, the greatest decrease occurring in triglycerides. In the plasma of 19-day fetuses, there is only one lipoprotein electrophoretic band which corresponds to the beta-lipoproteins of adults (low density lipoproteins, LDL) according to its mobility and floating characteristics, while in the fetus of 21 days there are also pre-beta-lipoproteins (VLDL). In the 1- and 5-day-old newborns, the four main lipoprotein fractions are already present as in the adults. The late appearance of VLDL and the lack of high density lipoproteins in the rat fetus determine various specific metabolic characteristics.