A vasopressin substrate radioimmunoassay (RIA) system is described for the measurement of the vasopressinase elaborated during pregnancy. Results show that vasopressinase activity increases in the sera of pregnant women throughout gestation. At term, under the in vitro conditions described, pregnancy serum degrades vasopressin (Pitressin) at a rate of 53 mU. per milliliter of serum per minute. Serum obtained from paired umbilical cord samples had an activity of 9.2 μU per milliliter per minute; amniotic fluid obtained at delivery had an activity of 16.6 μU per milliliter per minute. Pregnancy serum obtained from sheep, rabbits, and rats, ovine umbilical cord serum, and ovine amniotic fluid had no detectable vasopressinase activity. Normal human male serum, nonpregnant female serum, and serum from women taking contraceptive agents also exhibited no activity.