The mode of conjugation of urinary metabolites of injected 17β-estradiol-6,7-3H was studied in 10 nonpregnant human females. Estrone was the major identified constitutent of a fraction released from conjugation by solvolysis and this “estrone sulfate” accounted for 2–65% of total urinary estrone. In one instance, in an apparently normal young female, estrone-3-sulfate was directly identified as a quantitatively major metabolite, exceeding the glucosiduronates of both estrone and estriol. The considerable “sulfate conjugation” of estrone and of other estrogen metabolites in this subject was apparently a reproducible finding. Besides estrone, 2-methoxyestrone was regularly identified in the subjects studied, as a component of the fraction released by solvolysis and this conjugated form accounted for up to 63% of total urinary 2-methoxyestrone. Endogenous urinary “estrone sulfate” in the normal menstrual cycle varied considerably between subjects and accounted for 0–71% of total estrone. Other metabolites of labeled 17β-estradiol, including the latter itself, 16α-hydroxyestrone, 16-ketoestradiol-17β, 16-epiestriol and estriol, were less frequently identified following solvolysis of urine originally incubated with β-glucuronidase, i.e., as “sulfates” and/or double conjugates such as “sulfoglucosiduronates.” In 3 out of 11 studies 16α-hydroxyestrone and 16-ketoestradiol-17β were identified in this form (16–52% of total) but estriol conjugated in this way formed no more than 7% of total estriol.