Hypothalamic-pituitary-gonadal (HPG) axis is strongly implicated in the regulation of immune system. The objective was to determine the effects of immunocastration on splenic reproduction- and immunity-related gene expressions, and serum cytokine profiles in rams. Forty rams were randomly allocated into three groups: control (n = 14); surgically castrated (n = 13); or immunized (n = 13) against 100 μg D-Lys6-GnRH-tandem-dimer peptide conjugated to ovalbumin in Specol adjuvant at 6 months of age (with a booster 2 months later). Blood samples (for hormone and immune cytokine profiles) were collected at 1-month intervals until rams were slaughtered (10 months). Compared to intact controls, anti-GnRH immunization reduced (P < 0.05) serum concentrations of LH, FSH, and testosterone. Reduced testosterone abrogated its inhibitor feedback effect on the synthesis of GnRH in spleen, as evidenced by increased (P < 0.05) protein content and mRNA expressions of GnRH, and simultaneously decreased (P < 0.05) mRNA expressions of androgen receptor in spleen. In parallel with the increased GnRH production in spleen, the mRNA expressions of interleukin (IL)-2, IL-4, IL-6 and tumor necrosis factor alpha (TNF-α) as well as lymphocyte marker CD4, CD8 and CD19 molecules were increased (P < 0.05) in spleen. Consistently, serum concentrations of IL-2, IL-4, IL-6, TNF-α were increased (P < 0.05) in rams following immunization. Similarly, deprivation of testosterone by surgical castration also increased (P < 0.05) GnRH and thus immune cytokine expressions in spleen. Collectively, our data suggested that immunocastration increased GnRH production in spleen by abrogating the inhibitory feedback effects from testosterone, consequently improving the immune markers of spleen and serum immune cytokines in rams.