The time course of free oestradiol and oestrone as well as of total (free and conjugated) oestrone was determined in plasma of women after oral ingestion of 3.75 mg conjugated equine oestrogens, or 9.74 micromol of oestrone sulfate, oestrone, and oestradiol, respectively. All these oestrogen preparations led to transiently increased plasma oestrogen levels which fell to normal values after 48 h. The main difference observed between administration of free oestrone or oestradiol and of conjugated oestrogens (oestrone sulfate or equine oestrogens) was a much more protracted influx of oestrogens from the intestine into the plasma compartment, with a tendency to more sustained plasma levels, if conjugated oestrogens were administered. There was a consistent discontinuity in plasma oestrogen levels 10--12 h after oral ingestion of all the preparations examined indicating enterohepatic circulation. Comparison of "areas under the curve" obtained with the present preparations to similar previous studies on ethinyl oestradiol indicated that the bioavailability of the non-ethinyl oestrogens is by more than one order of magnitude less than that of ethinyl oestradiol after oral administration. The ratio of oestrone/oestradiol was the highest with free oestrone and similar among the other three preparations indicating an increased metabolism of sulfoconjugated oestrone to oestradiol after oral application when compared with free oestrone.