Nutrition-induced changes in testicular size in Merino rams appear to involve both GnRH-dependent and -independent pathways. This hypothesis was tested by feeding mature Merino rams that had been actively immunized against BSA or GnRH conjugated to BSA a diet that maintained initial body weight or the same diet supplemented daily with 1.5 kg of lupin grain. Blood was sampled every 20 min for 24 h on days-1, 19 and 70 relative to the change in diet. The plasma was used to assess the effects of treatments on changes in LH, FSH and testosterone concentrations. In the group immunized against BSA, FSH increased in lupin-supplemented rams compared with maintenance-fed rams, while LH and testosterone were not affected by diet. In comparison, the concentrations of LH, FSH and testosterone were significantly lower in the group immunized against GnRH than in rams immunized against BSA, but none of these endocrine variables was affected by nutrition. With both immunization treatments, the testes were significantly larger in lupin-supplemented than in maintenance-fed rams. In the group immunized against BSA, this difference was caused by testicular growth in lupin-supplemented rams, whereas in the group immunized against GnRH, lupin supplementation effectively maintained testicular mass, rather than allowed the regression observed in maintenance-fed rams. In conclusion, differences in testicular growth that were induced by dietary treatments in rams immunized against GnRH were not associated with changes in gonadotrophin or testosterone secretion. This supports the hypothesis that part of the effect of nutrition on testicular growth is independent of changes in GnRH secretion. The differences in testicular size observed in control rams were of similar magnitude to those observed in treated rams, but associated with large differences in plasma FSH concentrations, suggesting that this hormone plays an important role in this effect.