Abstract: Changes in some biochemical and haematological indices including serum protein, haemoglobin, cholesterol, lipid peroxidation, white blood cells, Glutathione-s-transferase, Superoxide dismutase, Alanine transaminase, Aspartate transaminase and Alkaline phosphatase were investigated in male rats fed with a diet supplemented with leaves of Gnetum africanum. The histological changes on the liver, intestines and testes were also examined. The long term feeding of the Gnetum africanum-supplemented diet caused significant increases (p<0.05) in weight, haemoglobin and white blood cells. There were also significant increases (p<0.05) in Glutathione-s-transferase and Superoxide dismutase enzymes. However, Gnetum africanum-supplemented diet caused a significant reduction (p<0.05) in serum protein and lipid peroxidation. The liver enzymes namely Alanine transaminase, Aspartate transaminase and Alkaline phosphatase were unaffected while the reduction in cholesterol was not significant. Histologically, the liver hepatocytes and hepatic plates were respectively elongated and enlarged while the intestinal mucosa showed elongated villi and enlarged submucosa. There were however no histological changes on the testes.