Serum leucine aminopeptidase activity measured by its action on hydrolysis of L-leucyl-beta-naphthylamide was increased progressively as pregnancy advanced. Angiotensinase activity determined by bioasaay showed parallel increase during normal pregnancy. 2. The effects of heat treatment and L-methionine on the activity of angiotensinase were compared with those on leucine aminopeptidase activity in pregnancy serum.