Lynestrenol is a potent oral pregestagen. The oral activities in the Clauberg and Corner & Allen tests are of the same order as that of subcutaneously administered progesterone. No effect was found in the tests for deciduomagenic activity in the mouse and for the maintenance of pregnancy in the rat. Lynestrenol has a marked inhibitory action on ovulation in the rat. This effect can be enhanced by low doses of oestrogen, which themselves are inactive in this respect. The main cause of this effect seems to be an inhibition of the development of the follicles. Lynestrenol is a weak oestrogen with an activity of about 1–2% of that of ethinyloestradiol. Its effects on fertilized ova in the rat are in agreement with what could be expected from its oestrogenic activity. There is little or no effect on nidation. Lynestrenol is a weak androgen, the oral activity in the rat amounting to 1/3–1/4 of that of methyltestosterone. 5. Acute and chronic toxicity studies demonstrate the absence of any general toxicity.