Activity of arylamidases toward 6 beta-naphthylamides of L-amino acids was studied in the blood serum from 110 healthy women between the 6th and 36th weeks of pregnancy. A regular increase in the enzymatic activity was demonstrated toward all substrates under examination, particularly in the presence of alanyl-, leucyl- and lysyl-beta-naphthylamides, most pronounced in the last trimester of gestation. A correlation between oxytocinase and arylamidases activity was demonstrated, suggesting a possibility of using the enzymatic measurement as a diagnostic test in the normal course of pregnancy.