The effect of i.v. injection of 200 @tg gonadotropin releasing hormone(GnRU) on plasma LU, FSH andestradiol-1 713 (E2-1 7i3) was investigated in18 prepuberal Holstein heifers ateither 3,6 or 9 months of age. GnRH caused increased levels of both LU and FSU within 20 min postinjection. Both LU and FSU plasma concentrations reached peak values by 20 mm following treatment in 3-and 9-month-old heifers, but in 6-month-old animals neither hormone reached maximum con centration until 2 h postinjection. Levels of both gonadotropins were not different with age. The profiles of both gonadotropins were similar following GnRU treatment (r = 0.86), but the relative magnitude of LU response (30-fold) was greater than FSU response (8-fold). FSU response was less variable in9-month-old heifers thaninyounger animals.Concentration of plasma E2-1713was not altered by GnRH treatment by 6 h postinjection. It appears thattheprepuberal heifer iscapable ofreleasing large quantities ofbothLU andFSH and that this response is not different at 3, 6 or 9 months of age. However, it does not appear that the GnRU-induced rise in plasma gonadotropins is sufficient stimulus to promote increased E2-1713 production within 6 h after GnRU administration in the prepuberal heifer.

The effects of unilateral castration (UC) and induced unilateral cryptorchidism (CR) on plasma hormones and testis anatomy were studied in 36 Holstein bulls altered at either 3, 6 or 9 mo of age (n = 12). Plasma hormone concentrations were determined in six samples collected at hourly intervals on d 0, 1, 3, 7, 14 and 30, and then at monthly intervals through 6 mo after gonadal manipulation. Although plasma testosterone (T) showed a transient decrease (P less than .05) immediately after treatment, mean plasma concentrations of luteinizing hormone (LH) and T were unaffected by UC or CR over the 6-mo period (P greater than .05). Both hormones increased (P less than .05) in concentration with advancing age. Plasma follicle stimulating hormone (FSH) concentration was greater (P less than .05) in UC than in intact (IN) bulls overall, while FSH in CR bulls did not differ (P greater than .05) from either group. At slaughter, 11 mo after gonadal alteration, mean testis weight, ratio of testis weight to body weight and mean testis sperm cell numbers were increased (P less than .05) in UC bulls compared with mean testis values in intact (IN) bulls. Unilateral castration increased (P less than .05) seminiferous tubuler diameter and seminiferous epithelial cell height from basement membrane to the border of the lumen, but did not alter the ratio of tubuler to interstitial space within the testis. Seminiferous tubuler diameter and epithelial cell height were increased (P less than .05) in CR compared with IN bulls. Unilateral gonadal alteration at 3 mo of age caused a greater (P less than .05) hypertrophy of the scrotal testis in both UC and CR bulls than alteration at 6 or 9 mo of age. Results indicate that unilateral gonadal disruption is followed by rapid compensation in testis T production, little change in systemic LH and a rapid increase in secretion of FSH in the bull within those ages investigated. Further, UC elicits a greater compensatory hypertrophy than CR and the pituitary-testis endocrine axis is more responsive to alteration at 3 mo than at 6 or 9 mo of age in the bull.

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