A group of 17 healthy women, of whom 9 had a predisposition to diabetes, were studied for five consecutive menstrual cycles with regard to the effect of an oral combined contraceptive containing 0.5 mg norgestrel and 0.05 mg ethinylestradiol (Eugynon®). The fasting concentration in the serum of triglycerides, free fatty acids and insulin together with the respone values during i.v. glucose toleracne for free fatty acids and insulin were estiamted. The tests were performed at midcyle n the cycle prior to, in two ycles during and in the second cycle after cessation of the drug administratino. The drug was administered daily from the fifth to twentyfifth day of the cycle in the two cycles. Thus each subject served as her own control. Statistical analysis of the results showed a significant increase in the fasting values of teh serum triglycerides during hormonal administration in those predisposed to diabetes, but unchanged values in those not so predisposed. The fasting values of the serum free fatty acids were, for the series as a whole, significantly lower in the second cycle of administration. There was no significant difference with regard to the predisposition to diabetes, but a tendeny to unchanged values in those predisposed to diabetes and lower values in those not predisposed. The fasting values of the serum insulin showed no changes and no difference with regard to predisposition to diabetes. The response to i.v. glucose injection showed, for the free fatty acids, no significant changes during hormonal administration, but the fasting values also had a tendency to unchanged values for thsoe predisposed and lower values for those not predisposed to diabetes, during hormonal administratino. The response values of serum insulin were, for the series as a whole, significantly higher during hormonal administration, those predisposed to diabetes showed 60 min after glucose injection significantly higher values than those not so predisposed during hormonal administration. The results are discussed and compared to the findings of other investigators. Possible modes of action with regard to the increase in triglycerides in thsoe predisposed to diabetes are discussed. The study indicates, but does not prove, that in women predisposed to diabetes there is an increased risk of the development of hypertriglyceridaemia and consequent disease with the administration of the oral contraceptive studied.