SUMMARY: [6,7-3H]Oestrone was infused into virgin and 24-h post-partum lactating rabbits for a period of 1 h. At the end of the infusion period the concentration of free [6,7-3H]oestrone and [6,7-3H]oestradiol-17β was determined in plasma and in selected tissues (uterus, vagina, muscle, cerebral cortex, hypothalamus and anterior pituitary). Similar studies were carried out on virgin rabbits infused with [6,7-3H]oestradiol-17β. The tissues showed characteristic differences in the concentration of labelled oestradiol-17β and in the ratio labelled oestradiol-17β:oestrone, irrespective of whether the animal was infused with tritiated oestrone or oestradiol-17β. The uterus and vagina contained predominantly oestradiol-17β, while the oestradiol-17β:oestrone ratios of the pituitary and other tissues approximated to that of the plasma.