Newborn female guinea pigs treated with HCG or testosterone enantate at birth display a ‘precocious puberty’ i.e., advanced vaginal opening and normal estrous cycles. Animals treated with a high dose of HCG (1,500 RU) had smaller ovaries than the controls or those treated with 500 RU. All groups had corpora lutea in their ovaries. Hemispaying reaction was 89.8% in 1,500-RU animals, 67.2% in the 500-RU group, and 37.2% in the controls. It is concluded that the ‘ovarian refractory period’ in the newborn guinea pig is not present under our experimental conditions. The influence of the ovarian steroid secretion on these results is discussed.