The effects of nomegestrol acetate on circulating hormone levels, metabolic and hemostatic parameters and blood pressure were studied in 36 premenopausal women. The progestogen was administered from day 7 to 25 of the cycle during six cycles at a dosage (5 mg/d) known to inhibit ovulation. Analysis were performed before and in the third and sixth cycles. Estradiol and progesterone levels decreased significantly (p less than 0.001) during treatment. Body weight, fasting blood glucose and insulin, total HDL and LDL cholesterol, apolipoprotein B, fibrinogen and plasminogen did not change significantly. Triglycerides in the third cycle (p less than 0.05) and apolipoprotein A1 levels (p less than 0.01) in both periods of sampling decreased significantly. There was a significant increase in antithrombin III (p less than 0.01). These results indicate that nomegestrol acetate has no deleterious effect on blood glucose and lipids. The decrease in apolipoprotein A1 and increase in antithrombin III may be related either to the decrease in estradiol levels induced by the treatment or to the effect of the progestogen itself.