Ethynyloestradiol was administered to rabbits, guinea pigs and rats, and the concentration of the steroid in blood was measured by radioimmunoassay. In both rabbits and guinea pigs, levels of conjugated steroid were much higher than those of the freely extractable form. Whereas considerable amounts of steroid were present in a conjugated form in plasma 24 h after injection, none was present at this time in a freely extractable form. There were significant differences between young and adult rabbits and guinea pigs in the rate at which ethynyloestradiol was metabolized. The amounts present in the freely extractable form in rats were higher than in the other two species but no steroid was detected in the conjugated fraction. The results are compared with previous findings in humans.