The existence of a positive feedback of estrogen on the gonadotrophin-releasing mechanism at birth, was tested in the female guinea pig. Estradiol benzoate (100 ng during the first 5 days of life) or HCG (100 RU during the first 5 days of life) induced a true precocious puberty, with similar characteristics to the control animals (60% of the animals ovulated at puberty age: corpora lutea present at the ovaries). The uterus was larger in ovulating than in nonovulating animals. Reserpine (2.5 mg/kg) on 15, 17 and 19 days of age did not modify the precocious puberty induced by estradiol at birth. It is concluded that exogenous or endogenous estrogen (induced by exogenous gonadotrophin administration) have a positive feedback action on the newborn female guinea pig to release gonadotrophin and develop puberty. These results are very different than those observed in the rat and differences can be related to the stage of somatic and nervous maturation attained by the animals at birth.