The present study was conducted to elucidate the profile of circulating gonadotropins and gonadal hormones from birth to puberty and relationship between gonadal seasonality and hormonal secretion in both sexes of Thoroughbred horses. Spring-born colts (n=6) and fillies (n=9) were blood sampled weekly from jugular vein from birth to 60 weeks of age. Circulating FSH, LH, prolactin, testosterone, progesterone, estradiol-17β, and immunoreactive (ir)-inhibin were measured by radioimmunoassay. In both sexes, the steroid hormones levels were remarkably high at birth, rapidly dropped within a week and remained at the lower levels until the start of second spring after birth. Ir-inhibin was also high during the birth, remaining lowest during winter and again increasing towards the second summer. There was an increase in FSH concentration in foals during the first summer months after birth and in the next summer, the FSH concentration along with that of LH increased significantly. The seasonal increase in circulating prolactin was remarkable even in the first year, and no differences were noted between the two summers. These results clearly demonstrated that the hypothalamo-pituitary axis is already responsive to changes in photoperiod and secrete prolactin similar to adult horses, but pituitary gonadotrophs for FSH and LH secretion is less sensitive. When the values of these hormones in the second breeding season after birth were compared with adult values of the respective sex in the breeding season, no significant differences were observed, indicating that spring-born fillies and colts have already attained the stage of puberty at the second breeding season after birth.