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A total of ten boars were immunized with a vaccine against GnRH and the parameters for testicular function and N-retention compared with eight intact boars and eight barrows. The immunization of boars occurred at 10, 16 and 23 weeks of age. In six pigs from each group the nitrogen retention was determined at 20/21 weeks. Blood for testosterone determination was collected daily and fat biopsies for androstenone measurement every 2nd week from week 17 to 26 in all boars. After slaughter, testes size, backfat thickness and skatole concentrations in fat were determined. The immunized boars revealed significant lower (P≤0.001) testes weights (100.5 vs. 882.6 g), plasma testosterone concentrations (0.11 vs. 2.77 ng/ml) and fat androstenone concentrations (0.06 vs. 1.31 μg/g, week 26) compared to controls. Skatole concentrations also differed significantly between immunized boars and controls (14.4 vs. 109.2 ng/g fat). The immunized boars had a significantly poorer feed conversion rate (3.82 vs. 3.01 kg/kg) and nitrogen retention (22.97 vs. 31.24 g/day) compared to boars. Barrows did not differ significantly from the immunized boars. It is concluded that an early and repeated immunization against GnRH is highly effective for a complete androstenone suppression and a considerable reduction of skatole but it also abolishes the boar-specific anabolic potential. [Copyright &y& Elsevier]