The concentrations of hydrochlorothiazide (HCT) in maternal and umbilical cord plasma and in amniotic fluid were determined at partus in 10 pregnancies. The levels of HCT in umbilical cord plasma were 0.1--0.8 of that in maternal plasma. The concentrations of the diuretic in amniotic fluid were up to 5 and 19 times higher than in maternal and umbilical cord plasma, respectively.