In 139 new student nurses, a change of menstrual cycles after school entry was studied, recording their basal body temperature over more than 4 months. In some of these students, serum FSH, LH, prolactin, estradiol, and progesterone were assayed every day or every other day throughout the second cycle after entry. Forty-six out of 72 students with previously normal menstrual cycles changed the length of their cycles. In most of the cases with a change in menstrual cycle, LH seemed not to be released in adequate amounts judging by the peak of estrogen and ovulation did not occur; the lack of sufficient LH stimulation resulted in anovulatory shortened cycles, amenorrhea or prolonged ovulatory cycles, followed by delayed ovulation. Also, in these cases, corpus luteum insufficiency persisted even when the ovulation returned. Prolactin was not considered to be involved in the mechanism of environmental menstrual disorders because serum prolactin did not increase in the cases with menstrual change. It can be concluded that environmental change or stress influences LH-RH cells in the hypothalamic centers simultaneously with the stimulation of ACTH secretion, and suppresses ovulation by decreasing LH secretion, and causes the different types of menstrual disorders.