Effects of prolonged administration (7 days) of Tamoxifen (Tx: 100 micrograms/day), oestradiol-17 beta (E2; 1 microgram/day) and oestriol (E3; silastic capsules) on the uterine weight, protein, DNA and progesterone receptor (RP) content of prepubertal rats have been studied. It has been observed when compared with E2 that Tx induces a dissociated action after two days of administration; the RP content is still increasing when the other parameters are at a constant level. E3 has a similar action to E2.