Sexual maturation involves restraint of gonadal activity from birth to puberty, while somatic development continues. Fetal gonadal steroidogenesis is very important for the maintenance of pregnancy and foaling in mares. The present study was conducted to evaluate gonadal steroid serum levels in foals after birth. Five fillies born at Equília Stud Farm, Avaré, SP, Brazil, were studied. Blood samples were collected daily in the morning during the first week of life. Progesterone serum levels were measured by a commercial radioimmunoassay (RIA) kit and estradiol serum levels with a sensitized non-commercial RIA. At birth, both progesterone and estradiol serum levels were high (13.46 ñ 5.5 nmol/l and 7.95 ñ 1.5 nmol/l) and decreased to undetectable levels at the end of the first week of life. A negative correlation was found between fillie age and gonadal serum steroid concentration. Results show that fetal gonadal activity persists until birth, and decreases gradual and slowly during the first week of life, as reflected by steroid concentrations in the blood