A standardized 50 g glucose tolerance test was given twice to 192 women with regular menstrual cycle within an interval of some weeks. The data showed that higher blood sugar values and glucosuria occurred significantly more often in the second half of the menstrual cycle, which was true also for women using monophasic type contraceptives. However, the reproducibility of tests--performed twice in the second half of the menstrual cycle--was only 24%, as compared to about 60%, when the first test was performed during menstruation or in the first half between two bleedings. Therefore, the latter intervalls seem to be the most appropriate time to perform a glucose tolerance test on premenopausal women.