Abstract The absorption of 17α-ethynylsteroids was studied by an in situ procedure in the rat and in vivo in the rat, guinea-pig and rabbit. Absorption of ethynyloestradiol and norethisterone from rat intestinal loops in situ was rapid, more than 90% being absorbed within 1 h. Most of the absorbed steroid could be recovered in the bile showing an entero-hepatic circulation of both steroids. Whereas derivatives of norethisterone were absorbed as rapidly as norethisterone, ethynyloestradiol-3-sulphate was only slowly absorbed. The steroids in bile were present predominantly in a conjugated form and those present in intestinal washings, the gut wall and portal plasma were present mainly in a free state. Only small amounts of norethisterone were absorbed from the stomach but absorption from the large intestine was rapid. Absorption of norethisterone from oily solutions was slower than when administered in ethanol-saline using the in situ technique but in vivo there was no difference between the two formulations in the rat, guinea-pig and rabbit. Both ethynyloestradiol and its sulphate were rapidly absorbed by the guinea pig in vivo.