Abstract The biological responses of estriol (E 3) and of estriol-3-sulfate (Ea 3-S) in the fetal and newborn uteri of guinea pig were studied. After a treatment of E 3 (1 mg/kg/day) or E 3-S (1.4 mg/kg/day) to pregnant guinea pigs (49–60 days of gestation) for 6 days, both estrogens provoke a significant uterotrophic effect in the fetal uterus which increases in weight 1.8–2.5 times in relation to the non-treated animals. The stimulation of progesterone receptor (PR) is also very intense, 7–12 times in relation to the control animals. In another series of experiments newborn guinea pigs (2-day old) were treated with 100μg/animal of E 3 or 140 μg/animal of E 3-S for a short (2 days) or a long (12 days) period. Concerning the uterotrophic effect, the weight of the uterus increases 1.8–2.5 times (in relation to the non-treated animals) after a 2-day treatment and the effect continues to increase up to 4–5 times in the 12-day treated animals. In opposition to the fetal uterus, the effect on PR provoked by E 3 or E 3-S is very limited (only an increase of 1 time in relation to the non-treated animals); the effect is even less intense after the 12-day treatment. Histological studies show an intense hypertrophic effect particularly in the epithelium of the endometrium in the fetal and newborn uteri for both E 3 and E 3-S. In the newborns the effect is also intense on the epithelium of the uterine gland. It is concluded that: (1) Estriol and estriol sulfate are very active and with similar intensity on the uterine weight before and after birth; (2) The stimulatory effect on PR decreases very significantly after birth and after a long treatment; (3) E 3S can act as a potent hormonal precursor.