The changes in the biochemical composition of maternal plasma, fetal plasma and fetal extraembryonic fluid were studied in Wistar rats from Days 13 1/2 to 21 1/2 of gestation. During this period, there was an increase in maternal plasma volume and total blood volume. The total protein concentration decreased, but little change was observed in the protein pattern, total alpha-amino nitrogen, urea nitrogen and sodium concentration. In fetal plasma, total protein concentration increased progressively with a prominent increase in albumin near term. The albumin:globulin ratio was lower than that in maternal plasma because of the presence of two alpha-fetoproteins and four transferrins. The concentration of alpha-amino nitrogen was higher than that in maternal plasma but the urea nitrogen level did not change significantly. The fetal extraembryonic fluid reflected the changes in composition of fetal plasma, especially in the protein pattern, and concentrations of alpha-amino nitrogen and sodium. Total protein and urea nitrogen increased markedly in concentration near term but there was reduction in the fluid volume at this time.