The ovarian function of 20 healthy users of a levonorgestrel-releasing intrauterine device was studied by measurements of plasma estradiol, progesterone, and levonorgestrel concentrations. Eight subjects were amenorrheic, and 12 had regular menstrual bleeding. Seventy-five percent of the subjects in both groups had ovulatory cycles. There was no difference between the mean levonorgestrel concentrations in the amenorrheic and regularly menstruating subjects. Estradiol function was undisturbed in the amenorrheic subjects with ovulatory cycles. The effect of levonorgestrel as a cause of amenorrhea is thought to be mainly of local nature. The intrauterine administration of levonorgestrel had only an occasional and weak effect on ovarian function.