The experiment described in the paper focused on the effects of complementary and excess diet supplementation on feed consumption, body weight changes, and fat tissue accumulation in rats. Untreated wheat, corn grains, and barley grits in the original diet were isocalorically substituted with wheat flour, sucrose, and B-complex vitamins. Diet supplementation was found to significantly modify the level of feed intake. The lowest amount of feed (per unit body weight) was consumed by the rats fed the excess supplementation diet. In addition, males of that group showed the lowest (per unit feed weight) body weight gains (2.84±0.75 g) and the highest (0.430±0.100 g) accumulation of pericardial fat, significantly higher than that in the non-supplemented (0.053±0.012) and complementarily supplemented (0.086±0.029 g) groups; moreover, the accumulation of perivisceral fat (5.74±1.42 g) was significantly higher than in the non-supplemented group (8.789±1.832 g). On the other hand, the females on the excess supplementation diet showed slightly lower (per 100 g food consumed) body weight gains (2.01±0.77 g), compared to the females fed the modified, non-supplemented diet (2.23±0.80), their body weight gains being significantly higher than those recorded in the females on the complementary supplementation diet (1.15±0.70 g). The higher body weight gains in that group were accompanied by a significantly higher accumulation of pericardial (0.663±0.176 g) and perivisceral (12.79±2.40 g) fatty tissue as well as by an increased muscle tissue fat content (10.5%), compared to the females fed modified non-supplemented diet (0.057±0.018 g; 5.644±0.986g; 6.30%, respectively) and the complementary supplementation diet (0.048±0.011 g; 5.958±1.174 g; 5.24% , respectively).