This experiment examined the effects of immunizing against ACTH (adrenocorticotrophic hormone) and GnRH (hypothalamic peptide gonadotrophic releasing hormone) on growth and carcase composition of entire ram lambs from 6 to 35 weeks of age. There were four experimental groups (each of 17 animals) in a 2 x 2 factorial design: (1) a non-immunized control group, (2) immunized against ACTH, (3) immunized against GnRH and (4) immunized against both antigens. After booster injections, experimental lambs exhibited antibody titres to the two antigens presented, resulting in significantly lower cortisol and testosterone levels (P < 0.001) in peripheral blood. The fall in blood cortisol levels in response to ACTH immunization was later matched by a similar decline in control animals to a point where they were not different. In GnRH-immunized animals, antibodies disrupted the pituitary-gonadal axis, thereby impacting severely on testes development in growing lambs to the degree that by day 119 their testes were smaller than at the commencement of the experiment. Towards the end of the growth phase of the animals, GnRH-immunized lambs had significantly lower body weights at day 189 (P < 0.05) and day 204 (P < 0.01) while ACTH-immune lambs were not different in weight from control animals. Also, GnRH-immunized lambs were found to have greater GR measurement (P < 0.05). Although ACTH immunization was capable of lowering cortisol for a brief period during the current experiment, the results were variable and detrimental to the efficacy of the immuno-castration vaccine (GnRH).