The rate at which oestrogens reached their sites of action was investigated by the local application of dimethylstilboestrol at short intervals after the local application of each oestrogen. Receptor occupation appeared to be complete within 2–4 min. of the application of oestradiol-17β and of oestrone and in some cases with oestradiol within 1 min. However, such was the degree of variability between experiments that more precise estimates were not possible. These findings are discussed in relation to the mode of action of the oestrogens.