The objective was to determine effects of active immunization against GnRH on reproductive function in Tibetan rams. Peripubertal Tibetan rams (n = 30) were randomly and equally allocated into three groups: control (no treatment); surgically castrated; or immunized against 100-μg d-Lys6-GnRH-tandem-dimer peptide conjugated to ovalbumin in Specol adjuvant at 24 weeks of age (with a booster 8 weeks later). Blood samples (for antibody titers and hormone concentrations) were collected at 4-week intervals until rams were killed (40 weeks). Immunization triggered a good antibody response in all immunized rams (P < 0.01). Compared with intact controls, anti-GnRH immunization reduced (P < 0.01) serum concentrations of testosterone, inhibin A, LH, and FSH, and it induced testicular atrophy (suppression of spermatogenesis). Androstenone concentrations in fat tissues of GnRH-immunized rams were also rendered nondetectable (P < 0.001). Furthermore, mRNA expressions of GnRH receptor, LH-β, and FSH-β in the pituitary and of LH receptor, FSH receptor, and inhibin α and βA subunits in the testes were decreased in immunized rams compared with intact controls (P < 0.05). This was apparently the first report that active immunization against GnRH-tandem-dimer-ovalbumin conjugate in Specol adjuvant was an effective alternative to surgical castration for Tibetan rams under practical Tibetan plateau conditions.