The aim was to measure the ionic fluoride concentration in maternal plasma and in amniotic fluid after oral administration of different doses of sodium fluoride (NaF) to 121 pregnant women. They were divided into six groups, according to the dose administered; 0 for the control group and 0.25, 0.50, 0.75, 1.00 or 1.25 mg of F- for the others. The subjects were instructed to take the corresponding NaF dose both 24 and 3 h before amniocentesis. Amniotic fluid (5 ml) and venous blood (5 ml) were obtained from each subject. Ionic fluoride concentration was measured with an expanded-scale potentiometer and a selective fluoride electrode. The results showed that F- concentration in amniotic fluid and, presumably, in fetal circulation, was not significantly different in groups taking 0.25, 0.50, 0.75 or 1.00 mg/day of F-. The F- concentration in amniotic fluid of the 1.25 mg/day group was, however, significantly higher than in all the other groups.