Two experiments were conducted in order to determine the effects of estradiol (E2) on the development of the hypothalamic-pituitary-testicular axis in bull calves. In experiment 1, calves were assigned randomly to one of the following groups: 1) intact, 2) intact E2-treated, 3) castrated, or 4) castrated E2-treated. Treatments began when the calves were 7.5 wk of age and continued for 16.5 wk. Samples of blood were collected once a week from 3 to 14 wk of age and every 10 min for 6 hr at 8, 12 and 16 wk of age. Concentrations of E2 in plasma decreased between 3 and 4 wk of age and were further reduced by castration. Maximum concentrations of E2 (24.3 pg/ml) were observed 72 hr after insertion of E2 implants, however, plasma E2 stabilized at 5.9 pg/ml by 2 wk after insertion of E2 implants. Treatment with E2 eliminated the pulsatile secretion of LH in intact and castrated calves and retarded testicular growth. In experiment 2, calves were assigned to a control (n = 4) or E2-treated (n = 6) group. Implants of E2 were inserted at 7.5 wk of age. At 24 wk of age, calves were bled and then sacrificed to collect hypothalamic and pituitary tissues. Age-related changes in testicular weight and secretion of LH were blocked by E2. Neither the morphology nor the intensity of immunostaining of GnRH nerve cell bodies in the preoptic area (POA) were affected by E2. However, the density of GnRH fibers and beads in the stalk median eminence (SME), and concentrations of pituitary GnRH receptors were greater (P less than .01) in E2-treated compared to control calves. In addition, concentrations of norepinephrine (NE) in the SME were lower in E2-treated calves when compared to controls. Based on these observations, it is concluded that administration of E2 at 7.5 wk of age causes profound alterations in hypothalamic function including, changes in metabolism of NE and suppression of GnRH release.