Abstract.The results of two randomized prospective studies on metabolic changes induced in groups of healthy Young women by commercial oral contraceptives (OC) are described. In the first study, four low‐estrogen (30 μg or 35 μg ethinylestradiol) combined products were compared, while in the second, effects of four different formulations, each containing levonorgestrel plus ethinylestradiol, were determined. Results are presented for oral glucose tolerance and plasma insulin responses, serum total cholesterol, HDL‐cholesterol, triglycerides, coagulation factors I, VII, VIII, and X, plasminogen, anti‐thrombin III, and (for study no. two only) renin activity, substrate, and concentration. In the first study, products containing norethisterone or ethynodiol diacetate were associated with significant increases in serum triglycerides and total cholesterol, though HDL cholesterol fell. Coagulation and fibrinolytic factors showed similar changes. The product containing levonorgestrel showed the smallest alteration in all tests.In study 2, the effects of formulation were investigated. Most tests showed changes approximately in proportion to the dose of both steroidal components, though the estrogen appeared to exert the most pronounced effect. Minimal alterations in most tests occurred in women using the lowest fixed dose formulation (150 μg levonorgestrel + 30 μg ethinylestradiol), or the triphasic product. Significant effects on the renin system were seen with all four formulations; again, the two lowest dose preparations produced the smallest changes.Most of the biochemical changes observed were established during the first few cycles of oral contraceptive treatment, then remained approximately constant to the end of the present study periods. The effects appear to be determined by the estrogen and progestogen doses of the various products, and to interactions between the two