Within 8 min after the intravaginal administration of 25 pg of [3H]oestradiol ‘[3H E2] an equilibrium appeared to have been established between E2in the ‘nuclear’ and ‘soluble’ compartments of vaginal homogenates. When an initial injection of [[3H E2]was followed 4 or 64 min later by an intravaginal injection of unlabelled E2there was little effect on the retention of the initial dose at 4 min after the second dose and a highly significant decrease in retention at 64 min. However, the distribution of the initial dose between the two compartments was little affected at any time studied. When the second injection contained E2labelled with 3H or 14C an unexpectedly high proportion of this dose was retained at 4 min. With the [14C]E2 an increase in the proportion of the second dose in the nuclear compartment was observed 64 min after injection. The results suggested considerable recycling of E2 or the receptor complex between the two tissue compartmentsIt was not possible to differentiate significantly the sedimentation behaviour of the nuclear and soluble receptors when both were centrifuged in buffer containing O·4M KCI. The nuclear receptor showed no change in sedimentation coefficient between high salt(0·4M KCI) and low salt(O·O1M KCl)conditions.