The biological properties of 8-aza quinestrol (AQ) were compared to quinestrol (Q) and ethynyloestradiol (EE). AQ is an orally active oestrogenic steroid, which is taken up and released from body fat and therefore has some prolonged effects. Storage of AQ, however, was less than that reported for Q and, as a consequence, Q had considerably more prolonged action. AQwas less potent than either steroid in the immature rat uterotrophic assay. In the adult rat, AQ was less potent than Q but more potent than EE for prevention of pituitary-related responses (fertility, ovulation and ovarian hypertrophy after hemicastration). It was concluded that substitution of a nitrogen atom in the 8 position decreased both the biological and fat storage properties of Q.