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Dihydrospirorenone (DHSP; 6 beta,7 beta,15 beta,16 beta-dimethylen-3-oxo-17- alpha-pregn-4-en-21,17-carbolacton) is an aldosterone antagonist 8 times as potent as spironolactone in the rat. It is also a progestogen that suppresses ovulation in normal women at a daily dosage of 2 mg. The effects of this dosage on the renin-aldosterone system and sodium and potassium balances were investigated in two experiments. In study I, 12 healthy women received a diet with 100 mmol sodium and 60-70 mmol potassium per day from days 3-13 of their normal menstrual cycles. Six women took 2 mg DHSP; 6 others received placebo from days 8-13 of the cycle. Sodium excretion in the DHSP group rose from a mean of 79 to 98.5 +/- 8.3 mmol/day during medication. Placebo had no effect. The difference between average sodium excretion rates in subjects treated with DHSP or placebo was close to significance (P = 0.053). Potassium excretion did not change. Weight loss was slightly greater after DHSP than placebo treatment. PRA and plasma and urinary aldosterone rose significantly during DHSP medication. In study II, 12 women on a free diet were studied during a control and a treatment cyle. From days 5-25 of the second cycle, they took 2 mg DHSP (n = 6) or 1 mg cyproterone acetate. Both compounds suppressed ovulation and the rise in progesterone. During cycle 1, sodium excretion, PRA, and aldosterone were higher in the luteal than in the follicular phase, probably due to an antialdosterone effect of progesterone. DHSP reversed this pattern of natriuresis by inducing a significant early natriuresis and a rise in PRA and aldosterone. Cyproterone acetate only abolished differences in natriuresis between the follicular and luteal phases and the rise of PRA and plasma aldosterone in the luteal phase. We conclude that DHSP may be a suitable partner of ethinyl estradiol as a constituent of an oral contraceptive, since its progestogenic and antialdosterone profile is similar to that of progesterone. Other synthetic progestogens are devoid of an antialdosterone effect. The antialdosterone effect of DHSP may help prevent sodium retention and a rise in blood pressure in susceptible women.