It has been shown that a single dose of triamcinolone acetonide (TA) has an antiovulatory effect in baboons and humans when it is given at the onset of menstruation. To investigate the effect of TA on the function of the corpus luteum and succeeding menstrual cycles, 40 mg of TA was injected intramuscularly during the postovulatory period (between 1-3 days after ovulation) in ten normal menstruating women. Blood was taken from ovulatory stage to menstruation in each 3 or 4 days during the treated and control cycles, and was determined for LH, FSH, estradiol and progesterone by radioimmunoassay. Suppression of plasma LH (m most subjects) and slight suppression of plasma estradiol (in some subjects) were observed by postovulatory TA injection; however, no suppression of plasma progesterone was observed in 8 out of 10 women. The length of the treated luteal phase was not altered. In the succeeding menstrual cycle (the 2nd cycle), ovulation was not observed in all ten women, and a shortening of menstrual cycle was observed in 7 out of 10 women. In the 3rd cycle, the biphasic basal body temperature pattern (BBT) was observed in 4 out of 10 women, and in the 4th cycle, the biphasic BBT pattern was observed in 8 out of 10 women.These results infer that TA may affect the hypothalamic-pituitary axis, suppress gonadotropin secretion and subsequently impair estrogen secretion from the ovary; however, the possibility is not excluded that TA may directly affect ovarian function and induce de-creased estrogen secretion. Postovulatory TA injection may not suppress luteal function as judged by the plasma level of progesterone and length of the treated luteal phase. However, histological examination of the corpus luteum is necessary for its conclusion. Postovulatory TA injection blocked ovulation in all ten women in the succeeding menstrual cycle. This evidence infers that TA has long activity and/or that succeeding follicular development may commence just after ovulation. Investigations into effects on follicular maturation and ovulation by TA may provide important clues concerning these mechanisms in women.