Abstract. 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. Key words: Prolactin, Puberty, Seasonality, Thoroughbred (J. Reprod. Dev. 58: [522][523][524][525][526][527][528][529][530] 2012) E quines are seasonal breeders with activity being highest as days grow longer [1]. There is no univocal reporting of the age of puberty in horses, which has been reported to vary in the range of 7.8-37 months [2]. This variation can be attributed to differences in breeds [3][4][5], locales, management and puberty definition [6,7]. Furthermore, there have been few studies on Thoroughbreds [7][8][9], and most of them were conducted in southern hemisphere. Geographical location influences the season, and there have been fewer reports regarding puberty in horses from the northern hemisphere.There is a dearth of information on the characteristics of circulating gonadotropins and gonadal hormones in Thoroughbred foals immediately after birth through pre-pubertal age until puberty. Previous studies provided information on relatively shorter periods [10,11] of developmental periods after birth, and those studies focused on certain hormones [3,4,6] only. In seasonal breeders, there is evidence for the involvement of both photoperiodic cues and degree of maturity in the onset of puberty [8]. The photoperiodic information is thought to be conveyed to the reproductive and prolactin axis via changes in circulating concentra...

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