The objective of the present study was to evaluate the effect of different diets on the performance and development of internal organs of farm-raised fallow deer bucks. Supplemented deer had higher slaughter weight (P < 0.001) and their carcasses produced higher dressing percentage (P < 0.001). Significant differences attributable to the different dietary treatments were also observed in the weights of offal. As a consequence of better growth performance, the supplemented deer exhibited significantly higher weights of head (P = 0.001) and skin (P < 0.001). Moreover, the supplementation of barley to the feed ration significantly increased the weights of heart (P < 0.001), liver (P = 0.035), spleen (P = 0.002) and fatness characteristics (P < 0.001).