In Caucasian women, the length of the menstrual cycle is 28 days with a SD of 3.9 days. Many factors including diet modify menstrual periodicity. We have determined the effect of a meatless vegetarian diet in Caucasian women on the length of the menstrual cycle and the episodic and luteinizing releasing hormone-stimulated release of luteinizing hormone. The effect of a Western diet on the menstrual cycle in urban Black South African women was also studied. In Caucasian women, the vegetarian diet decreased (p less than or equal to 0.01) the pituitary response to luteinizing releasing hormone and decreased (p less than or equal to 0.05) the episodic release of luteinizing hormone. A Western diet increased (p less than or equal to 0.01) the duration of the follicular phase in Black South African women. Thus diet is a factor modifying the episodic release of gonadotrophins and follicular maturation.