The objective of this trial was to determine the pattern of declined of fat androstenone, the main causative chemical of boar taint, in tact male pigs after vaccination with a recombinant GnRH protein. Four groups of pigs were used. The primary vaccination was given as the pigs reached 100 kg. Weight gain, fat androstenone and blood samples assayed for testosterone and anti-GnRH antibodies were collected pre-second immunization and weekly thereafter until slaughter. All pigs were slaughtered 42 days after the second immunization and testicular weight, bulbourethral gland length, carcass weight and back fat depth were measured. There was no difference in carcass weights although live weights prior to slaughter were different between treatment groups. The immunocastrates, late castrates and boars had less backfat than barrows. As expected, mean testicular weight and bulbourethral glands length of boars at slaughter were greater than those of immunocastrates. GnRH titers were detected in all immunized animals 28 days after primary immunization. Within seven days after the second immunization or surgical castration, the mean serum testosterone concentrations of both the immunocastrates and the late castrates were significantly lower than those of boars. By Day 14, there was no difference between immunocastrates, late castrates and barrows. Serum testosterone concentrations remained suppressed until slaughter. Within seven days after surgical castration, mean fat androstenone concentrations in late castrates had dropped below the olfactory detection level for boar taint (0.5A