The plasma levels of progesterone and oestrogens were measured during treatment with an oral contraceptive containing 37.5 mug of ethinyl oestradiol and 1 mg of lynestrenol (= 3-desoxy-17alpha-ethinyl-19-nortestosterone) (Ovostat 1375, Organon, Oss, Holland). Blood samples were taken every day during 9 complete cycles in 5 healthy women. The plasma levels of progesterone were in the range of those found during the early follicular phase of the normal women. No ovulation seemed to have occurred. The mean plasma levels of oestrogens (oestradiol and oestrone) during treatment were below those found during early follicular phase of 34 normal cycles although overlapping of values occurred. The ratio between oestradiol and oestrone during treatment was 1:2 vs. 1:1 during the early follicular phase of the normal cycles. In two cycles treatment was started during the midcyclic rise of oestrogens. No rise of progesterone indicating corpus luteum formation was found. Despite the low amount of oestrogen in the drug the ovaries appeared to secrete minimal amounts of sex steroids. The lynestrenol content is likely to be partly responsible for the inhibition of ovulation and the additional suppression of steroidogenesis.