The changes in the concentrations of proteolytic enzyme inhibitors after the administration of hormones influencing the activity of the ovaries can cause an imbalance in the development of follicles. Important roles are attributed to serine proteases during the development of follicle, ovulation, and activity of the corpus luteum. In order to find out the changes in the trypsin inhibiting activities (TIA) of the blood plasma and cervical mucus after the administration of PGF2 alpha (100 + 100 micrograms in January and 250 + 250 micrograms in May on the 0th and 11th day) and after the single administration of 750 I. U. PMSG on the 10th day, a 14-day trial was performed to investigate the circadian changes. TIA was determined by means of slowing down the hydrolysis of the synthetic substrate N-alpha-tosyl-p-nitro-anilide (TAPA) by bovine trypsin. When the blood plasma TIA is expressed as the percent of the control values, a large drop is observed on the 11th and 12th days of the January trial (total as well as low-molecular TIA). In the May experiment this drop was recorded only on the 11th day in total TIA (c TIA); low-molecular TIA (n TIA), on the other hand, increased, particularly after the administration of PMSG. The average values of plasma TIA's, recorded in the course of the 14-day trial, increased after the administration of PGF2 alpha (c TIA increased to 109.8 +/- 22.1%, n TIA to 135.42 +/- 54.0% in January; c TIA to 112.59 +/- 50.5%, n TIA to 109.48 +/- 55.9% in May). After the administration of PMSG the TIA values remained higher than the controls (c TIA 107.13 +/- 24.0%, n TIA 133.40 +/- 52.1% in January; c TIA 113.87 +/- 51.3%, n TIA 118.33 +/- 55.6% in May). The average TIA values of the cervical mucus increased after PGF2 alpha administration to 3.06 +/- 1.7% in comparison with the control value of 2.59 +/- 1.1%, and also after the administration of PMSG to 2.82 +/- 1.9% (January, TIA per 1 mg of mucus).