Summary. Plasma concentrations of ethynyl oestradiol and norethindrone in women were measured by radioimmunoassays after oral administration of 50 pg and 1000 pg respectively. The maximum values were obtained 1 h after administration. The calculated half-life was 6\ m=1/ 2\ h for ethynyl oestradiol and 7 h for norethindrone. At most 2\ p=n-\ 3% of the administered dose was present in the plasma at 1 h and had decreased to about 0\ m=. \ 5 % by 24 h.