The objective of this research was to determine the correlation among hormone levels at 2- and 6-month of age and diameter and histology of the testis in Angus x Charolais young bulls at 6 months (n= 15). At two months, two basal blood samples were collected at 1-h intervals. After the last collection, the animals received 5 mg of GnRH and 1.5 and 3.0 h later, new samples were collected. At 6 months, the young bulls were treated with 1 mg of estradiol benzoate (E) and received 10 mg GnRH twelve hours later. After five days, the young bulls received 10 mg GnRH, but without pre-treatment with estradiol. In both cases, blood samples were collected 1.5 and 3.0 h after GnRH injection. Immediately before the GnRH and estradiol treatments, two basal samples were collected. The animals were surgically castrated at six months. Concentrations of GnRH-stimulated FSH at two months were related to testis diameter, number of Sertoli cells and A1 spermatogonia per testis (r=.65 to .80). The associations between FSH at two months and number of seminiferous tubule sections without germ cells (r=.59) and tubules with round spermatids (r= -.55) were also significant. The correlations between the basal LH and steroids and testis parameters were not significant. At 6 months, the basal levels of FSH did not alter 12 h after estradiol, but the FSH concentrations stimulated with GnRH were 2.5-fold higher when estradiol was used than when GnRH was used. Regression models with Sertoli cell numbers in function of FSH after GnRH showed that when GnRH + estradiol was used, a greater R² (.43) was obtained in comparison to GnRH injection only (.30). Thus, peripheral concentrations of FSH are potential indicators of testicular development at the prepubertal phase.
testis; FSH; estradiol; Sertoli cell; young bulls