During oral supplementation of 5 mg folic acid and 1 microgram vitamin B12, the daily folate levels in plasma, red cells and midtrimester amniotic fluid were significantly higher in ten pregnant women during the 15th-18th week of menstrual age as compared to ten non-supplemented women serving as controls. In the control group as well as in the supplemented group, the folate concentrations in amniotic fluid were found to be lower than in the corresponding maternal plasma and red cells. Of all women investigated there was a significant positive correlation between maternal plasma folate concentrations and amniotic fluid levels (r = 0.72, p less than 0.001) and no correlation between red cell folate concentrations and amniotic fluid levels (r = 0.30, p = 0.22). Oral supplementation of vitamin B12 did not elevate maternal blood concentrations and amniotic fluid levels. Vitamin B12 levels in amniotic fluid in this group and controls were always higher than in plasma. These data suggest that the orally administrated dosages of folic acid unlike those of vitamin B12 augment both plasma, red cells and amniotic fluid levels.