In an effort to determine the influence of hypothyroidism on the hypothalamopituitary-ovarian axis plasma levels of luteinizing hormone (LH), follicle stimulating hormone (FSH), oestrogen and progesterone were measured daily for 10 to 14 consecutive days in five women with hypothyroidism. In all subjects investigated the levels of all the hormones measured were low in comparison with the values in euthyroid women. FSH/LH ratios were, however, higher in hypothyroid women than in both phases of the menstrual cycle in euthyroid controls. It is postulated that changes in the FSH/LH ratio may be related to the low levels of oestrogen and probably to failure of ovulation in hypothyroid women.