Serum progesterone levels were measured by RIA in 23 Thai women during the first 3 of a 6-month treatment period of taking 30 ug d-Norgestrel daily and again for a 3-month period after discontinuing its use in order to determine the frequency of ovulation. During the period of use, 11 out of the 23 subjects showed no evidence of ovulation while only 1 subject failed to ovulate during the post-treatment stage. The median time for the post-treatment resumption of ovulation was 20 days with the earliest occurring before day 10 and the latest around day 85. The frequency of ovulation in women who did ovulate during the treatment period appeared to be depressed with an average of 1.8 ovulations/subject as compared to 2.4 during the post-treatment. Menstrual bleeding patterns were not significantly altered.