Ageratum conyzoides Linn. is known as plant growth inhibitor. To know whether the plant can inhibit growth of animal also, male albino rats were divided into two groups. First group of animals took normal diet while the second group took a mixture of normal diet and powdered leaves of Ageratum conyzoides Linn. In both the two groups growth of rats was checked up to 40 days. On 20th and 40th days of experiment blood pyruvate level was measured. Intestines of rats were also assayed for pyruvate dehydrogenase and transketolase activity. It was found out that growth of rats which took powdered leaves of Ageratum conyzoides Linn. with normal diet was stunted. Rats developed symptom of anorexia and became weak. Blood pyruvate level was elevated and intestinal pyrvate dehydrogenase activity was lowered. Pyruvate thus could not be converted to acetyl coenzyme A needed for generation of energy. Lack of energy caused growth inhibition in rats. Further, intestinal transketolase activity was found decreased. Due to this in pentose phosphate pathway there was insufficient production of NADPH and pentose sugars needed for synthesis of nucleic acid. This also caused growth inhibition in rats.