Diesterase activity was studied in human uterine secretions of normal women and in those under treatment for sterility or contraception. Endometrial secretions were obtained from 78 patients and the material divided into four groups: normal women, under different estrogens; with D-Norgestrel treatment (daily and post-coital) and patients with IUD Lippes D. The mean concentration of free choline delivered by the GPC diesterase in the normal group was 777 +/- 128 mug/ml (SD 286). Under hormonal treatment an increase of diesterase activity was observed. D-Norgestrel post-coital produced a fall of the enzymatic activity between 180 to 420 minutes. The uninterrupted use of D-Norgestrel (30 gammas daily) produced a loss of diesterase activity in 80% of cases studied. The use of IUD (Lippes D) did not modify the enzymatic activity in this group.