3H]Norethisterone was administered to women 1, 3 and 24 h before hysterectomy, when samples of blood, adipose tissue and tissues from the reproductive tract were obtained. At 1 h the highest concentrations of radioactivity were found in endometrium and stroma (0\m=.\77 and 0\m=.\78% respectively of the dose per 100 g tissue), the values in other tissues varying from 0\m=.\13to 0\m=.\39 %. By 3 h there was a decrease in the concentration in all tissues except adipose tissue and plasma and at this time the range of concentrations in all tissues, apart from plasma (0\m=.\40%), were similar (0\m=.\08 to 0\m=.\19%). There was a further decrease by 24 h. Mean values for the amount of radioactivity present in the tissues at 1 h were: uterus, 0\m=.\55%; adipose tissue, 21\m=.\6%;blood, 10\m=.\1%.By 24 h these values had decreased to 0\m=.\1, 7\m=.\0and 6\m=.\6%respectively. In adipose tissue most of the radioactivity was present in the freely extractable state at all times studied, whereas in myometrium and cervix the amount of radioactivity freely extractable at 1 h (77%) had decreased by 24 h (39\m=.\0%). In plasma the amount of conjugated radioactivity increased from 54% at 1 h to 88% by 24 h. Whereas at 1 h most metabolites in plasma were conjugated as sulphates, by 24 h glucuronide conjugates were also important. In myometrial tissue 17\m=.\5to 47\m=.\8% of the radioactivity was associated with the nuclear fraction.