A combination of 3H-labelled oestriol-3-sulphate-16-glucosiduronate (OE3-3S,16Gl) and 14C-labelled oestriol-16-glucosiduronate (OE3-16Gl) was infused over a 6 hour period to one woman at midpregnancy and to another woman with missed abortion in the 4th month of gestation. The urinary metabolites were isolated and identified. Sixty-four per cent of the 3H- and about 80 % of the 14C-labelled material was excreted in the urine within 24 hours. Some 20 % of the 3H-labelled, but only 2–3 % of the 14C-labelled material recovered was OE3-3S,16Gl. Approximately one half of the 3H-, but less than one-third of the 14C-labelled material was oestriol-3-glucosiduronate (OE3-3Gl). On the other hand, more than two-thirds of the 14C-labelled material, but less than one-third of the 3H-labelled was OE3-16Gl. The urinary elimination of OE3-3S,16Gl was much slower than that of OE3-16Gl and resembled closely the behaviour of oestriol-3-sulphate (OE3-3S) in this respect. The amounts of endogenous OE3-3S,16Gl and OE3-16Gl reaching the circulation each hour were calculated from their urinary excretion and from the relationship of infused and excreted labelled OE3-3S,16Gl and OE3-16Gl. On the basis of these estimates it is concluded that OE3-3S,16Gl is a quantitatively important constituent of pregnancy plasma, but not of pregnancy urine. It is suggested that OE3-3S,16Gl is formed mainly, if not entirely from OE3-16Gl, most probably in the liver and perhaps also in the gastrointestinal tract. A general scheme is presented, describing the over-all metabolism of conjugated oestriol.