the amino acid analysis and preliminary toxicological effect of Garcinia mangostana seed cake on albino rats have been investigated in order to determine its suitability as an additive in feed supplement. The proximate analysis of the seed cake showed that it had a high carbohydrate value of 71.02 ± 0.79% but low protein, moisture, fat and ash contents of 8.09 ± 0.21%, 10.42 ± 1.12%, 2.13 ± 0.13% and 1.8 ± 0.02% respectively.The seed cake also contains manganese, calcium, potassium, sodium, magnesium, iron, copper and zinc. The result of the phytochemical analysis revealed that Garcinia mangostana seed cake contains tannins, flavonoids and alkaloids. Among the seventeen amino acid determined in the defatted seed, glutamate gave the highest concentration of 10.94 g/100g. During the rat experiment which lasted for six weeks, both the control and test rats appeared to suffer no toxicological effect and weekly monitoring showed good physical appearance. There was no significant difference between the control and test groups for the haematology and histopathology examination of sections of the heart, liver, kidney, spleen, lung, intestine and brain result.